diff --git a/app/models/entry.rb b/app/models/entry.rb
index 84c2719f2e..1599f4ad47 100644
--- a/app/models/entry.rb
+++ b/app/models/entry.rb
@@ -301,6 +301,30 @@ def protected_from_sync?
     excluded? || user_modified? || import_locked?
   end
 
+  # Bulk-marks the entries of the given transactions as user-modified so a
+  # later provider sync won't overwrite them (issue #1977). Used by merchant
+  # merge/convert/unlink flows, which reassign merchant_id directly on
+  # transactions and must protect that manual change from being reverted.
+  #
+  # Accepts a Transaction relation (preferred — the selection runs as a
+  # subquery so large merges/unlinks don't materialize ids or hit SQL
+  # parameter limits) or an explicit array of ids.
+  #
+  # @param transactions [ActiveRecord::Relation, Array<String>] Transactions or their ids
+  # @return [void]
+  def self.mark_user_modified_for_transactions!(transactions)
+    entryable_ids =
+      if transactions.is_a?(ActiveRecord::Relation)
+        transactions.select(:id)
+      else
+        ids = Array(transactions).compact.uniq
+        return if ids.empty?
+        ids
+      end
+
+    where(entryable_type: "Transaction", entryable_id: entryable_ids).update_all(user_modified: true)
+  end
+
   # Marks entry as user-modified after manual edit.
   # Called when user edits any field to prevent provider sync from overwriting.
   #
diff --git a/app/models/merchant/merger.rb b/app/models/merchant/merger.rb
index 7baa78e678..4c57e4844b 100644
--- a/app/models/merchant/merger.rb
+++ b/app/models/merchant/merger.rb
@@ -39,8 +39,15 @@ def merge!
 
     Merchant.transaction do
       source_merchants.each do |source|
+        scope = family.transactions.where(merchant_id: source.id)
+
+        # Protect the manual reassignment from being reverted on the next
+        # provider sync (issue #1977). Must run before the merchant_id update
+        # so the scope still matches the source merchant.
+        Entry.mark_user_modified_for_transactions!(scope)
+
         # Reassign family's transactions to target
-        family.transactions.where(merchant_id: source.id).update_all(merchant_id: target_merchant.id)
+        scope.update_all(merchant_id: target_merchant.id)
 
         # Delete FamilyMerchant, keep ProviderMerchant (it may be used by other families)
         source.destroy! if source.is_a?(FamilyMerchant)
diff --git a/app/models/provider_merchant.rb b/app/models/provider_merchant.rb
index 5cfb2fdf9d..fbfba77353 100644
--- a/app/models/provider_merchant.rb
+++ b/app/models/provider_merchant.rb
@@ -15,8 +15,14 @@ def convert_to_family_merchant_for(family, attributes = {})
         website_url: attributes[:website_url].presence || website_url
       )
 
+      scope = family.transactions.where(merchant_id: id)
+
+      # Protect the manual reassignment from being reverted on the next
+      # provider sync (issue #1977). Must run before the merchant_id update.
+      Entry.mark_user_modified_for_transactions!(scope)
+
       # Update only this family's transactions to point to new merchant
-      family.transactions.where(merchant_id: id).update_all(merchant_id: family_merchant.id)
+      scope.update_all(merchant_id: family_merchant.id)
 
       family_merchant
     end
@@ -37,7 +43,13 @@ def generate_logo_url_from_website!
   # Does NOT delete the ProviderMerchant since it may be used by other families.
   # Tracks the unlink in FamilyMerchantAssociation so it shows as "recently unlinked".
   def unlink_from_family(family)
-    family.transactions.where(merchant_id: id).update_all(merchant_id: nil)
+    scope = family.transactions.where(merchant_id: id)
+
+    # Protect the manual unlink from being reverted on the next provider sync
+    # (issue #1977). Must run before the merchant_id is nulled.
+    Entry.mark_user_modified_for_transactions!(scope)
+
+    scope.update_all(merchant_id: nil)
 
     # Track that this merchant was unlinked from this family
     association = FamilyMerchantAssociation.find_or_initialize_by(family: family, merchant: self)
diff --git a/test/models/merchant/merger_test.rb b/test/models/merchant/merger_test.rb
new file mode 100644
index 0000000000..cc50b4950b
--- /dev/null
+++ b/test/models/merchant/merger_test.rb
@@ -0,0 +1,35 @@
+require "test_helper"
+
+class Merchant::MergerTest < ActiveSupport::TestCase
+  include EntriesTestHelper
+
+  setup do
+    @family = families(:dylan_family)
+    @target = merchants(:netflix)
+    @source = merchants(:amazon)
+  end
+
+  # Regression: issue #1977. Merging merchants reassigns merchant_id via
+  # update_all; without flagging the entries as user_modified, the next
+  # provider sync reverts the merge.
+  test "merge! flags reassigned transactions as user_modified" do
+    entry = create_transaction(merchant: @source)
+    assert_not entry.user_modified?
+
+    Merchant::Merger.new(family: @family, target_merchant: @target, source_merchants: [ @source ]).merge!
+
+    entry.reload
+    assert_equal @target.id, entry.entryable.merchant_id, "transaction is reassigned to the target merchant"
+    assert entry.user_modified?, "merged transaction's entry must be flagged so provider sync won't revert it"
+  end
+
+  test "merge! does not touch entries of unrelated merchants" do
+    other = create_transaction(merchant: @target)
+    assert_not other.user_modified?
+
+    Merchant::Merger.new(family: @family, target_merchant: @target, source_merchants: [ @source ]).merge!
+
+    other.reload
+    assert_not other.user_modified?, "transactions already on the target are untouched"
+  end
+end
diff --git a/test/models/provider_merchant_test.rb b/test/models/provider_merchant_test.rb
new file mode 100644
index 0000000000..0f15aeaa18
--- /dev/null
+++ b/test/models/provider_merchant_test.rb
@@ -0,0 +1,37 @@
+require "test_helper"
+
+class ProviderMerchantTest < ActiveSupport::TestCase
+  include EntriesTestHelper
+
+  setup do
+    @family = families(:dylan_family)
+    @provider_merchant = ProviderMerchant.create!(name: "Acme Synced", source: "plaid")
+  end
+
+  # Regression: issue #1977. Converting a synced merchant to a family merchant
+  # reassigns merchant_id via update_all; the entries must be flagged so the
+  # next provider sync doesn't revert the conversion.
+  test "convert_to_family_merchant_for flags reassigned transactions as user_modified" do
+    entry = create_transaction(merchant: @provider_merchant)
+    assert_not entry.user_modified?
+
+    family_merchant = @provider_merchant.convert_to_family_merchant_for(@family)
+
+    entry.reload
+    assert_equal family_merchant.id, entry.entryable.merchant_id
+    assert entry.user_modified?, "converted transaction's entry must be flagged so provider sync won't revert it"
+  end
+
+  # Regression: issue #1977. Unlinking a synced merchant nulls merchant_id;
+  # without the flag the next sync re-links it.
+  test "unlink_from_family flags affected transactions as user_modified" do
+    entry = create_transaction(merchant: @provider_merchant)
+    assert_not entry.user_modified?
+
+    @provider_merchant.unlink_from_family(@family)
+
+    entry.reload
+    assert_nil entry.entryable.merchant_id
+    assert entry.user_modified?, "unlinked transaction's entry must be flagged so provider sync won't re-link it"
+  end
+end