diff --git a/sqlglot/dialects/bigquery.py b/sqlglot/dialects/bigquery.py
index c0adae18f5..36f7478ae7 100644
--- a/sqlglot/dialects/bigquery.py
+++ b/sqlglot/dialects/bigquery.py
@@ -17,6 +17,7 @@
     date_add_interval_sql,
     datestrtodate_sql,
     build_formatted_time,
+    extract_week_unit_info,
     filter_array_using_unnest,
     if_sql,
     inline_array_unless_query,
@@ -209,6 +210,40 @@ def _ts_or_ds_diff_sql(self: BigQuery.Generator, expression: exp.TsOrDsDiff) ->
     return self.func("DATE_DIFF", expression.this, expression.expression, unit)
 
 
+def _generate_bq_datetime_diff_unit(self: BigQuery.Generator, expression: exp.Expression) -> str:
+    """
+    Generate unit for *_DIFF functions, converting Week expressions to BigQuery syntax.
+
+    Canonical form -> BigQuery syntax:
+    - Week(SUNDAY) -> WEEK (BigQuery's default)
+    - Week(MONDAY) -> WEEK(MONDAY) (preserved during round-trip)
+    - Var(ISOWEEK) -> ISOWEEK (preserved as-is)
+
+    """
+    unit = expression.args.get("unit")
+
+    # Preserve ISOWEEK/WEEKISO as-is.
+    # Handle ISOWEEK/WEEKISO Var expressions
+    if isinstance(unit, exp.Var) and unit.name.upper() in ("ISOWEEK", "WEEKISO"):
+        return self.sql(unit)
+
+    week_info = extract_week_unit_info(unit)
+
+    if week_info:
+        day_name, _ = week_info
+        if day_name == "SUNDAY":
+            return "WEEK"
+
+        if isinstance(unit, exp.WeekStart):
+            # Preserve WeekStart expressions as-is
+            return self.sql(unit)
+
+        # Convert to WEEK(day) for all other cases
+        return self.sql(exp.Week(this=exp.var(day_name)))
+
+    return self.sql(unit_to_var(expression))
+
+
 def _unix_to_time_sql(self: BigQuery.Generator, expression: exp.UnixToTime) -> str:
     scale = expression.args.get("scale")
     timestamp = expression.this
@@ -242,6 +277,44 @@ def _build_datetime(args: t.List) -> exp.Func:
     return exp.TimestampFromParts.from_arg_list(args)
 
 
+def _normalize_week_unit(unit: t.Optional[exp.Expression]) -> t.Optional[exp.Expression]:
+    """
+    In BigQuery, plain WEEK defaults to Sunday-start weeks.
+    Normalize plain WEEK to WEEK(SUNDAY) to preserve the semantic in the AST for correct cross-dialect transpilation.
+    """
+    if isinstance(unit, exp.Var) and unit.name.upper() == "WEEK":
+        return exp.Week(this=exp.var("SUNDAY"))
+
+    # This branch is necessary for the WEEK keyword in DATE_DIFF(..., WEEK) to be normalized.
+    if isinstance(unit, exp.Column) and not unit.table and isinstance(unit.this, exp.Identifier):
+        if unit.name.upper() == "WEEK":
+            return exp.Week(this=exp.var("SUNDAY"))
+
+    return unit
+
+
+def build_date_time_diff_with_week_normalization(
+    exp_class: t.Type[E],
+) -> t.Callable[[t.List], E]:
+    """
+    Factory for *_DIFF functions that normalizes plain WEEK units to WEEK(SUNDAY).
+
+    These functions have signature: FUNC(expr1, expr2, date_part)
+    where date_part is at argument index 2.
+
+    Supports: DATE_DIFF, DATETIME_DIFF, TIME_DIFF, TIMESTAMP_DIFF
+    """
+
+    def _builder(args: t.List) -> E:
+        return exp_class(
+            this=seq_get(args, 0),
+            expression=seq_get(args, 1),
+            unit=_normalize_week_unit(seq_get(args, 2)),
+        )
+
+    return _builder
+
+
 def _build_regexp_extract(
     expr_type: t.Type[E], default_group: t.Optional[exp.Expression] = None
 ) -> t.Callable[[t.List], E]:
@@ -551,6 +624,7 @@ class Parser(parser.Parser):
             "CONTAINS_SUBSTR": _build_contains_substring,
             "DATE": _build_date,
             "DATE_ADD": build_date_delta_with_interval(exp.DateAdd),
+            "DATE_DIFF": build_date_time_diff_with_week_normalization(exp.DateDiff),
             "DATE_SUB": build_date_delta_with_interval(exp.DateSub),
             "DATE_TRUNC": lambda args: exp.DateTrunc(
                 unit=seq_get(args, 1),
@@ -1092,7 +1166,7 @@ class Generator(generator.Generator):
             exp.CTE: transforms.preprocess([_pushdown_cte_column_names]),
             exp.DateAdd: date_add_interval_sql("DATE", "ADD"),
             exp.DateDiff: lambda self, e: self.func(
-                "DATE_DIFF", e.this, e.expression, unit_to_var(e)
+                "DATE_DIFF", e.this, e.expression, _generate_bq_datetime_diff_unit(self, e)
             ),
             exp.DateFromParts: rename_func("DATE"),
             exp.DateStrToDate: datestrtodate_sql,
diff --git a/sqlglot/dialects/dialect.py b/sqlglot/dialects/dialect.py
index b3a583c454..1e68e7ac1f 100644
--- a/sqlglot/dialects/dialect.py
+++ b/sqlglot/dialects/dialect.py
@@ -832,6 +832,26 @@ class Dialect(metaclass=_Dialect):
         "CENTURIES": "CENTURY",
     }
 
+    # Mapping of week unit names to (start_day, iso_dow) tuples
+    # iso_dow follows ISO 8601 day-of-week numbering (Monday=1, ..., Sunday=7)
+    WEEK_UNIT_SEMANTICS = {
+        "WEEK": ("SUNDAY", 7),
+        "ISOWEEK": ("MONDAY", 1),
+        "WEEKISO": ("MONDAY", 1),
+    }
+
+    # Days of week to ISO 8601 day-of-week numbers
+    # ISO 8601 standard: Monday=1, Tuesday=2, Wednesday=3, Thursday=4, Friday=5, Saturday=6, Sunday=7
+    WEEK_START_DAY_TO_DOW = {
+        "MONDAY": 1,
+        "TUESDAY": 2,
+        "WEDNESDAY": 3,
+        "THURSDAY": 4,
+        "FRIDAY": 5,
+        "SATURDAY": 6,
+        "SUNDAY": 7,
+    }
+
     # Specifies what types a given type can be coerced into
     COERCES_TO: t.Dict[exp.DataType.Type, t.Set[exp.DataType.Type]] = {}
 
@@ -1705,6 +1725,50 @@ def map_date_part(part, dialect: DialectType = Dialect):
     return part
 
 
+def extract_week_unit_info(
+    unit: t.Optional[exp.Expression], dialect: DialectType = None
+) -> t.Optional[t.Tuple[str, int]]:
+    """
+    Extract week unit information from AST node.
+
+    This helper provides a unified way to handle week units across dialects.
+
+    Args:
+        unit: The unit expression (Var for ISOWEEK/WEEKISO, Week, or WeekStart)
+        dialect: Dialect used to access WEEK_UNIT_SEMANTICS and WEEK_START_DAY_TO_DOW mappings.
+
+    Returns:
+        Tuple of (day_name, iso_dow) where iso_dow is ISO 8601 day number (Monday=1, Sunday=7),
+        or None if not a week unit or if day is dynamic (not a constant).
+
+        Examples:
+            Week(Var('SUNDAY')) → ('SUNDAY', 7)
+            Var('ISOWEEK') → ('MONDAY', 1)
+            Column('week') → None (dynamic, not a constant)
+
+    """
+    dialect_instance = Dialect.get_or_raise(dialect)
+
+    # Handle plain Var expressions for ISOWEEK/WEEKISO only
+    if isinstance(unit, exp.Var):
+        unit_name = unit.name.upper()
+        if unit_name in ("ISOWEEK", "WEEKISO"):
+            week_info = dialect_instance.WEEK_UNIT_SEMANTICS.get(unit_name)
+            if week_info:
+                return week_info
+        return None
+
+    # Handle Week/WeekStart expressions with explicit day
+    if isinstance(unit, (exp.Week, exp.WeekStart)):
+        day_var = unit.this
+        if isinstance(day_var, exp.Var):
+            day_name = day_var.name.upper()
+            dow_value = dialect_instance.WEEK_START_DAY_TO_DOW.get(day_name)
+            return (day_name, dow_value) if dow_value is not None else None
+
+    return None
+
+
 def no_last_day_sql(self: Generator, expression: exp.LastDay) -> str:
     trunc_curr_date = exp.func("date_trunc", "month", expression.this)
     plus_one_month = exp.func("date_add", trunc_curr_date, 1, "month")
diff --git a/sqlglot/dialects/duckdb.py b/sqlglot/dialects/duckdb.py
index 4698df93e7..bd18529450 100644
--- a/sqlglot/dialects/duckdb.py
+++ b/sqlglot/dialects/duckdb.py
@@ -22,6 +22,7 @@
     datestrtodate_sql,
     no_datetime_sql,
     encode_decode_sql,
+    extract_week_unit_info,
     build_formatted_time,
     no_comment_column_constraint_sql,
     no_time_sql,
@@ -250,9 +251,77 @@ def _implicit_datetime_cast(
     return arg
 
 
+def _build_week_trunc_expression(date_expr: exp.Expression, start_dow: int) -> exp.Expression:
+    """
+    Build DATE_TRUNC expression for week boundaries with custom start day.
+
+    Args:
+        date_expr: The date expression to truncate
+        start_dow: ISO 8601 day-of-week number (Monday=1, ..., Sunday=7)
+
+    DuckDB's DATE_TRUNC('WEEK', ...) aligns weeks to Monday (ISO standard).
+    To align to a different start day, we shift the date before truncating.
+
+    Shift formula: Sunday (7) gets +1, others get (1 - start_dow)
+    Examples:
+        Monday (1): shift = 0 (no shift needed)
+        Tuesday (2): shift = -1 (shift back 1 day) ...
+        Sunday (7): shift = +1 (shift forward 1 day, wraps to next Monday-based week)
+    """
+    # Calculate shift: Sunday is special case, others use formula
+    shift_days = 1 if start_dow == 7 else 1 - start_dow
+
+    if shift_days == 0:
+        # No shift needed for Monday-based weeks
+        return exp.DateTrunc(unit=exp.var("WEEK"), this=date_expr)
+
+    # Shift date to align week boundaries with the desired start day
+    shifted_date = exp.DateAdd(
+        this=date_expr,
+        expression=exp.Interval(this=exp.Literal.string(str(shift_days)), unit=exp.var("DAY")),
+    )
+
+    return exp.DateTrunc(unit=exp.var("WEEK"), this=shifted_date)
+
+
 def _date_diff_sql(self: DuckDB.Generator, expression: exp.DateDiff) -> str:
+    """
+    Generate DATE_DIFF SQL for DuckDB using DATE_TRUNC-based week alignment.
+
+    DuckDB's DATE_DIFF doesn't support WEEK(day) units. We transpile these to
+    DATE_TRUNC-based calculations when the day is a compile-time constant.
+
+    Non-literal week units (e.g., columns, placeholders) cannot be transpiled safely
+    and are passed through as-is, following the pattern used elsewhere in sqlglot
+
+    """
     this = _implicit_datetime_cast(expression.this)
     expr = _implicit_datetime_cast(expression.expression)
+    unit = expression.args.get("unit")
+
+    # Extract week start day; returns None if day is dynamic (column/placeholder)
+    week_start = extract_week_unit_info(unit)
+    if week_start and this and expr:
+        _, start_dow = week_start
+
+        # For Monday-based weeks (ISO standard), use DuckDB's native WEEK unit
+        if start_dow == 1:
+            return self.sql(
+                exp.DateDiff(this=this, expression=expr, unit=exp.Literal.string("WEEK"))
+            )
+
+        # For other week start days, use DATE_TRUNC-based calculation
+        # Build truncated week boundary expressions
+        truncated_this = _build_week_trunc_expression(this, start_dow)
+        truncated_expr = _build_week_trunc_expression(expr, start_dow)
+
+        # Calculate week difference
+        day_diff = exp.DateDiff(
+            this=truncated_this, expression=truncated_expr, unit=exp.Literal.string("DAY")
+        )
+        result = exp.IntDiv(this=day_diff, expression=exp.Literal.number(7))
+
+        return self.sql(result)
 
     return self.func("DATE_DIFF", unit_to_str(expression), expr, this)
 
diff --git a/tests/dialects/test_bigquery.py b/tests/dialects/test_bigquery.py
index 151dd46b57..48d683f797 100644
--- a/tests/dialects/test_bigquery.py
+++ b/tests/dialects/test_bigquery.py
@@ -3240,6 +3240,107 @@ def test_week(self):
             "EXTRACT(WEEK(THURSDAY) FROM CAST('2013-12-25' AS DATE))",
         )
 
+        # BigQuery → DuckDB transpilation tests for DATE_DIFF with week units
+        # Test WEEK(MONDAY) - Monday-based week (optimized to use native DuckDB WEEK)
+        self.validate_all(
+            "DATE_DIFF('2024-01-15', '2024-01-08', WEEK(MONDAY))",
+            write={
+                "bigquery": "DATE_DIFF('2024-01-15', '2024-01-08', WEEK(MONDAY))",
+                "duckdb": "DATE_DIFF('WEEK', CAST('2024-01-08' AS DATE), CAST('2024-01-15' AS DATE))",
+            },
+        )
+        # Test WEEK(SUNDAY) - Sunday-based week (normalized to WEEK)
+        self.validate_all(
+            "DATE_DIFF('2024-01-15', '2024-01-08', WEEK(SUNDAY))",
+            write={
+                "bigquery": "DATE_DIFF('2024-01-15', '2024-01-08', WEEK)",
+                "duckdb": "DATE_DIFF('DAY', DATE_TRUNC('WEEK', CAST('2024-01-08' AS DATE) + INTERVAL '1' DAY), DATE_TRUNC('WEEK', CAST('2024-01-15' AS DATE) + INTERVAL '1' DAY)) // 7",
+            },
+        )
+        # Test WEEK(SATURDAY) - Saturday-based week calculation
+        self.validate_all(
+            "DATE_DIFF('2024-01-15', '2024-01-08', WEEK(SATURDAY))",
+            write={
+                "bigquery": "DATE_DIFF('2024-01-15', '2024-01-08', WEEK(SATURDAY))",
+                "duckdb": "DATE_DIFF('DAY', DATE_TRUNC('WEEK', CAST('2024-01-08' AS DATE) + INTERVAL '-5' DAY), DATE_TRUNC('WEEK', CAST('2024-01-15' AS DATE) + INTERVAL '-5' DAY)) // 7",
+            },
+        )
+        # Test WEEK - Default Sunday-based week calculation
+        self.validate_all(
+            "DATE_DIFF('2024-01-15', '2024-01-08', WEEK)",
+            write={
+                "bigquery": "DATE_DIFF('2024-01-15', '2024-01-08', WEEK)",
+                "duckdb": "DATE_DIFF('DAY', DATE_TRUNC('WEEK', CAST('2024-01-08' AS DATE) + INTERVAL '1' DAY), DATE_TRUNC('WEEK', CAST('2024-01-15' AS DATE) + INTERVAL '1' DAY)) // 7",
+            },
+        )
+        # Test WEEK - Saturday to Sunday boundary (critical test for Sunday-start weeks)
+        # In BigQuery: Saturday -> Sunday crosses week boundary = 1 week
+        # Without fix: DuckDB treats as Monday-start weeks = 0 weeks (both in same week)
+        self.validate_all(
+            "DATE_DIFF('2024-01-07', '2024-01-06', WEEK)",
+            write={
+                "bigquery": "DATE_DIFF('2024-01-07', '2024-01-06', WEEK)",
+                "duckdb": "DATE_DIFF('DAY', DATE_TRUNC('WEEK', CAST('2024-01-06' AS DATE) + INTERVAL '1' DAY), DATE_TRUNC('WEEK', CAST('2024-01-07' AS DATE) + INTERVAL '1' DAY)) // 7",
+            },
+        )
+        # Test ISOWEEK - ISO 8601 Monday-based week (optimized to use native DuckDB WEEK)
+        self.validate_all(
+            "DATE_DIFF('2024-01-15', '2024-01-08', ISOWEEK)",
+            write={
+                "bigquery": "DATE_DIFF('2024-01-15', '2024-01-08', ISOWEEK)",
+                "duckdb": "DATE_DIFF('WEEK', CAST('2024-01-08' AS DATE), CAST('2024-01-15' AS DATE))",
+            },
+        )
+        # Test with DATE literal - Explicit DATE casting (optimized for Monday-based week)
+        self.validate_all(
+            "DATE_DIFF(DATE '2024-01-15', DATE '2024-01-08', WEEK(MONDAY))",
+            write={
+                "bigquery": "DATE_DIFF(CAST('2024-01-15' AS DATE), CAST('2024-01-08' AS DATE), WEEK(MONDAY))",
+                "duckdb": "DATE_DIFF('WEEK', CAST('2024-01-08' AS DATE), CAST('2024-01-15' AS DATE))",
+            },
+        )
+
+        # Test Negative difference with WEEK(SUNDAY) - verifies argument order swapping
+        self.validate_all(
+            "DATE_DIFF(DATE '2024-01-01', DATE '2024-01-15', WEEK(SUNDAY))",
+            write={
+                "bigquery": "DATE_DIFF(CAST('2024-01-01' AS DATE), CAST('2024-01-15' AS DATE), WEEK)",
+                "duckdb": "DATE_DIFF('DAY', DATE_TRUNC('WEEK', CAST('2024-01-15' AS DATE) + INTERVAL '1' DAY), DATE_TRUNC('WEEK', CAST('2024-01-01' AS DATE) + INTERVAL '1' DAY)) // 7",
+            },
+        )
+        # Test Negative difference with ISOWEEK - verifies argument order swapping (optimized)
+        self.validate_all(
+            "DATE_DIFF(DATE '2024-01-01', DATE '2024-01-15', ISOWEEK)",
+            write={
+                "bigquery": "DATE_DIFF(CAST('2024-01-01' AS DATE), CAST('2024-01-15' AS DATE), ISOWEEK)",
+                "duckdb": "DATE_DIFF('WEEK', CAST('2024-01-15' AS DATE), CAST('2024-01-01' AS DATE))",
+            },
+        )
+        # Test Year boundary crossing (negative difference)
+        self.validate_all(
+            "DATE_DIFF(DATE '2023-12-25', DATE '2024-01-08', WEEK(SUNDAY))",
+            write={
+                "bigquery": "DATE_DIFF(CAST('2023-12-25' AS DATE), CAST('2024-01-08' AS DATE), WEEK)",
+                "duckdb": "DATE_DIFF('DAY', DATE_TRUNC('WEEK', CAST('2024-01-08' AS DATE) + INTERVAL '1' DAY), DATE_TRUNC('WEEK', CAST('2023-12-25' AS DATE) + INTERVAL '1' DAY)) // 7",
+            },
+        )
+        # Test Year boundary crossing (positive difference)
+        self.validate_all(
+            "DATE_DIFF(DATE '2024-01-08', DATE '2023-12-25', WEEK(SUNDAY))",
+            write={
+                "bigquery": "DATE_DIFF(CAST('2024-01-08' AS DATE), CAST('2023-12-25' AS DATE), WEEK)",
+                "duckdb": "DATE_DIFF('DAY', DATE_TRUNC('WEEK', CAST('2023-12-25' AS DATE) + INTERVAL '1' DAY), DATE_TRUNC('WEEK', CAST('2024-01-08' AS DATE) + INTERVAL '1' DAY)) // 7",
+            },
+        )
+        # Test DAY unit with negative difference - verifies non-week argument swapping
+        self.validate_all(
+            "DATE_DIFF(DATE '2024-01-01', DATE '2024-01-15', DAY)",
+            write={
+                "bigquery": "DATE_DIFF(CAST('2024-01-01' AS DATE), CAST('2024-01-15' AS DATE), DAY)",
+                "duckdb": "DATE_DIFF('DAY', CAST('2024-01-15' AS DATE), CAST('2024-01-01' AS DATE))",
+            },
+        )
+
     def test_approx_qunatiles(self):
         self.validate_identity("APPROX_QUANTILES(foo, 2)")
         self.validate_identity("APPROX_QUANTILES(DISTINCT foo, 2 RESPECT NULLS)")