diff --git a/packages/zod/src/zod4/converter.processed.test.ts b/packages/zod/src/zod4/converter.processed.test.ts
index 8feee8522..fc6d8032e 100644
--- a/packages/zod/src/zod4/converter.processed.test.ts
+++ b/packages/zod/src/zod4/converter.processed.test.ts
@@ -18,4 +18,10 @@ testSchemaConverter([
     input: [true, { type: 'string' }],
     output: [false, {}],
   },
+  {
+    name: 'preprocess(x => x, z.string())',
+    schema: z.preprocess(x => x, z.string()),
+    input: [true, { type: 'string' }],
+    output: [true, { type: 'string' }],
+  },
 ])
diff --git a/packages/zod/src/zod4/converter.ts b/packages/zod/src/zod4/converter.ts
index d25852a7f..9347b6242 100644
--- a/packages/zod/src/zod4/converter.ts
+++ b/packages/zod/src/zod4/converter.ts
@@ -519,7 +519,15 @@ export class ZodToJsonSchemaConverter implements ConditionalSchemaConverter {
 
           case 'pipe': {
             const pipe = schema as $ZodPipe
-            return this.#convert(options.strategy === 'input' ? pipe._zod.def.in : pipe._zod.def.out, options, lazyDepth, structureDepth)
+            return this.#convert(
+              // prefer out schema when in schema is preprocess/transform
+              options.strategy === 'input' && pipe._zod.def.in._zod.def.type !== 'transform'
+                ? pipe._zod.def.in
+                : pipe._zod.def.out,
+              options,
+              lazyDepth,
+              structureDepth,
+            )
           }
 
           case 'readonly': {