diff --git a/packages/openapi-typescript/src/transform/schema-object.ts b/packages/openapi-typescript/src/transform/schema-object.ts
index 9c5725126..d821dfea0 100644
--- a/packages/openapi-typescript/src/transform/schema-object.ts
+++ b/packages/openapi-typescript/src/transform/schema-object.ts
@@ -92,72 +92,79 @@ export function transformSchemaObjectWithComposition(
   if (
     Array.isArray(schemaObject.enum) &&
     (!("type" in schemaObject) || schemaObject.type !== "object") &&
-    !("properties" in schemaObject) &&
-    !("additionalProperties" in schemaObject)
+    !("properties" in schemaObject)
   ) {
-    // hoist enum to top level if string/number enum and option is enabled
-    if (
-      options.ctx.enum &&
-      schemaObject.enum.every((v) => typeof v === "string" || typeof v === "number" || v === null)
-    ) {
-      let enumName = parseRef(options.path ?? "").pointer.join("/");
-      // allow #/components/schemas to have simpler names
-      enumName = enumName.replace("components/schemas", "");
-      const metadata = schemaObject.enum.map((_, i) => ({
-        name: schemaObject["x-enum-varnames"]?.[i] ?? schemaObject["x-enumNames"]?.[i],
-        description: schemaObject["x-enum-descriptions"]?.[i] ?? schemaObject["x-enumDescriptions"]?.[i],
-      }));
-
-      // enums can contain null values, but dont want to output them
-      let hasNull = false;
-      const validSchemaEnums = schemaObject.enum.filter((enumValue) => {
-        if (enumValue === null) {
-          hasNull = true;
-          return false;
+    const hasAdditionalProperties = "additionalProperties" in schemaObject && !!schemaObject.additionalProperties;
+
+    if (!hasAdditionalProperties || (schemaObject.type === "string" && hasAdditionalProperties)) {
+      // hoist enum to top level if string/number enum and option is enabled
+      if (
+        options.ctx.enum &&
+        schemaObject.enum.every((v) => typeof v === "string" || typeof v === "number" || v === null)
+      ) {
+        let enumName = parseRef(options.path ?? "").pointer.join("/");
+        // allow #/components/schemas to have simpler names
+        enumName = enumName.replace("components/schemas", "");
+        const metadata = schemaObject.enum.map((_, i) => ({
+          name: schemaObject["x-enum-varnames"]?.[i] ?? schemaObject["x-enumNames"]?.[i],
+          description: schemaObject["x-enum-descriptions"]?.[i] ?? schemaObject["x-enumDescriptions"]?.[i],
+        }));
+
+        // enums can contain null values, but dont want to output them
+        let hasNull = false;
+        const validSchemaEnums = schemaObject.enum.filter((enumValue) => {
+          if (enumValue === null) {
+            hasNull = true;
+            return false;
+          }
+
+          return true;
+        });
+        const enumType = tsEnum(enumName, validSchemaEnums as (string | number)[], metadata, {
+          shouldCache: options.ctx.dedupeEnums,
+          export: true,
+          // readonly: TS enum do not support the readonly modifier
+        });
+        if (!options.ctx.injectFooter.includes(enumType)) {
+          options.ctx.injectFooter.push(enumType);
         }
+        const ref = ts.factory.createTypeReferenceNode(enumType.name);
+
+        const finalType: ts.TypeNode = hasNull ? tsUnion([ref, NULL]) : ref;
 
-        return true;
-      });
-      const enumType = tsEnum(enumName, validSchemaEnums as (string | number)[], metadata, {
-        shouldCache: options.ctx.dedupeEnums,
-        export: true,
-        // readonly: TS enum do not support the readonly modifier
-      });
-      if (!options.ctx.injectFooter.includes(enumType)) {
-        options.ctx.injectFooter.push(enumType);
+        return applyAdditionalPropertiesToEnum(hasAdditionalProperties, finalType, schemaObject);
       }
-      const ref = ts.factory.createTypeReferenceNode(enumType.name);
-      return hasNull ? tsUnion([ref, NULL]) : ref;
-    }
-    const enumType = schemaObject.enum.map(tsLiteral);
-    if ((Array.isArray(schemaObject.type) && schemaObject.type.includes("null")) || schemaObject.nullable) {
-      enumType.push(NULL);
-    }
 
-    const unionType = tsUnion(enumType);
+      const enumType = schemaObject.enum.map(tsLiteral);
+      if ((Array.isArray(schemaObject.type) && schemaObject.type.includes("null")) || schemaObject.nullable) {
+        enumType.push(NULL);
+      }
 
-    // hoist array with valid enum values to top level if string/number enum and option is enabled
-    if (options.ctx.enumValues && schemaObject.enum.every((v) => typeof v === "string" || typeof v === "number")) {
-      let enumValuesVariableName = parseRef(options.path ?? "").pointer.join("/");
-      // allow #/components/schemas to have simpler names
-      enumValuesVariableName = enumValuesVariableName.replace("components/schemas", "");
-      enumValuesVariableName = `${enumValuesVariableName}Values`;
+      const unionType = applyAdditionalPropertiesToEnum(hasAdditionalProperties, tsUnion(enumType), schemaObject);
+
+      // hoist array with valid enum values to top level if string/number enum and option is enabled
+      if (options.ctx.enumValues && schemaObject.enum.every((v) => typeof v === "string" || typeof v === "number")) {
+        let enumValuesVariableName = parseRef(options.path ?? "").pointer.join("/");
+        // allow #/components/schemas to have simpler names
+        enumValuesVariableName = enumValuesVariableName.replace("components/schemas", "");
+        enumValuesVariableName = `${enumValuesVariableName}Values`;
+
+        const enumValuesArray = tsArrayLiteralExpression(
+          enumValuesVariableName,
+          oapiRef(options.path ?? ""),
+          schemaObject.enum as (string | number)[],
+          {
+            export: true,
+            readonly: true,
+            injectFooter: options.ctx.injectFooter,
+          },
+        );
 
-      const enumValuesArray = tsArrayLiteralExpression(
-        enumValuesVariableName,
-        oapiRef(options.path ?? ""),
-        schemaObject.enum as (string | number)[],
-        {
-          export: true,
-          readonly: true,
-          injectFooter: options.ctx.injectFooter,
-        },
-      );
+        options.ctx.injectFooter.push(enumValuesArray);
+      }
 
-      options.ctx.injectFooter.push(enumValuesArray);
+      return unionType;
     }
-
-    return unionType;
   }
 
   /**
@@ -446,7 +453,7 @@ function transformSchemaObjectCore(schemaObject: SchemaObject, options: Transfor
     ("$defs" in schemaObject && schemaObject.$defs)
   ) {
     // properties
-    if (Object.keys(schemaObject.properties ?? {}).length) {
+    if ("properties" in schemaObject && schemaObject.properties && Object.keys(schemaObject?.properties).length) {
       for (const [k, v] of getEntries(schemaObject.properties ?? {}, options.ctx)) {
         if ((typeof v !== "object" && typeof v !== "boolean") || Array.isArray(v)) {
           throw new Error(
@@ -515,7 +522,7 @@ function transformSchemaObjectCore(schemaObject: SchemaObject, options: Transfor
     }
 
     // $defs
-    if (schemaObject.$defs && typeof schemaObject.$defs === "object" && Object.keys(schemaObject.$defs).length) {
+    if ("$defs" in schemaObject && typeof schemaObject.$defs === "object" && Object.keys(schemaObject.$defs).length) {
       const defKeys: ts.TypeElement[] = [];
       for (const [k, v] of Object.entries(schemaObject.$defs)) {
         const property = ts.factory.createPropertySignature(
@@ -584,3 +591,16 @@ function transformSchemaObjectCore(schemaObject: SchemaObject, options: Transfor
 function hasKey<K extends string>(possibleObject: unknown, key: K): possibleObject is { [key in K]: unknown } {
   return typeof possibleObject === "object" && possibleObject !== null && key in possibleObject;
 }
+
+function applyAdditionalPropertiesToEnum(
+  hasAdditionalProperties: boolean,
+  unionType: ts.TypeNode,
+  schemaObject: SchemaObject,
+) {
+  // If additionalProperties is true, add (string & {}) to the union
+  if (hasAdditionalProperties && schemaObject.type === "string") {
+    const stringAndEmptyObject = tsIntersection([STRING, ts.factory.createTypeLiteralNode([])]);
+    return tsUnion([unionType, stringAndEmptyObject]);
+  }
+  return unionType;
+}
diff --git a/packages/openapi-typescript/src/types.ts b/packages/openapi-typescript/src/types.ts
index 75d8f8c07..a1c55beeb 100644
--- a/packages/openapi-typescript/src/types.ts
+++ b/packages/openapi-typescript/src/types.ts
@@ -436,6 +436,7 @@ export type SchemaObject = {
   const?: unknown;
   default?: unknown;
   format?: string;
+  additionalProperties?: boolean | Record<string, never> | SchemaObject | ReferenceObject;
   /** @deprecated in 3.1 (still valid for 3.0) */
   nullable?: boolean;
   oneOf?: (SchemaObject | ReferenceObject)[];
diff --git a/packages/openapi-typescript/test/transform/schema-object/string.test.ts b/packages/openapi-typescript/test/transform/schema-object/string.test.ts
index 3ff497833..4e1c87257 100644
--- a/packages/openapi-typescript/test/transform/schema-object/string.test.ts
+++ b/packages/openapi-typescript/test/transform/schema-object/string.test.ts
@@ -119,6 +119,17 @@ describe("transformSchemaObject > string", () => {
         want: "string | null",
       },
     ],
+    [
+      "enum + additionalProperties",
+      {
+        given: {
+          type: "string",
+          enum: ["A", "B", "C"],
+          additionalProperties: true,
+        },
+        want: `("A" | "B" | "C") | (string & {})`,
+      },
+    ],
   ];
 
   for (const [testName, { given, want, options = DEFAULT_OPTIONS, ci }] of tests) {