diff --git a/.changeset/fix-temporal-isplainobject.md b/.changeset/fix-temporal-isplainobject.md
new file mode 100644
index 000000000..19f2f12f6
--- /dev/null
+++ b/.changeset/fix-temporal-isplainobject.md
@@ -0,0 +1,11 @@
+---
+'@tanstack/db': patch
+---
+
+fix(db): treat objects with `Symbol.toStringTag` as leaf values in `IsPlainObject`
+
+Temporal types (e.g. `Temporal.PlainDate`, `Temporal.ZonedDateTime`) have `Symbol.toStringTag` set to a string. Previously, `IsPlainObject` would return `true` for these types because they are objects and not in the `JsBuiltIns` union. This caused the `Ref<T>` mapped type to recursively walk Temporal methods, mangling them to `{}`.
+
+The fix adds a `T extends { readonly [Symbol.toStringTag]: string }` check before returning `true`, causing all class instances with `Symbol.toStringTag` (Temporal types, etc.) to be treated as leaf values with their types fully preserved.
+
+Fixes #1372
diff --git a/packages/db/src/query/builder/types.ts b/packages/db/src/query/builder/types.ts
index 6dce531f8..476438c13 100644
--- a/packages/db/src/query/builder/types.ts
+++ b/packages/db/src/query/builder/types.ts
@@ -862,6 +862,12 @@ export type Prettify<T> = {
 
 /**
  * IsPlainObject - Utility type to check if T is a plain object
+ *
+ * Returns `false` for:
+ * - Arrays (ReadonlyArray)
+ * - JavaScript built-ins (Date, Map, Set, etc.)
+ * - Objects with `Symbol.toStringTag` (class instances like Temporal types,
+ *   TypedArrays not already in JsBuiltIns, etc.) — these are not plain data objects
  */
 type IsPlainObject<T> = T extends unknown
   ? T extends object
@@ -869,7 +875,9 @@ type IsPlainObject<T> = T extends unknown
       ? false
       : T extends JsBuiltIns
         ? false
-        : true
+        : T extends { readonly [Symbol.toStringTag]: string }
+          ? false
+          : true
     : false
   : false