diff --git a/test/built-ins/Temporal/Instant/prototype/toString/timezone-string-unknown.js b/test/built-ins/Temporal/Instant/prototype/toString/timezone-string-unknown.js
new file mode 100644
index 00000000000..83aadd343e5
--- /dev/null
+++ b/test/built-ins/Temporal/Instant/prototype/toString/timezone-string-unknown.js
@@ -0,0 +1,26 @@
+// Copyright (C) 2026 Rudolph Gottesheim. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.instant.prototype.tostring
+description: Unrecognized time zone identifiers are not valid input for a time zone
+features: [Temporal]
+---*/
+
+const instance = new Temporal.Instant(0n);
+
+const unknown = ["Mars/Olympus_Mons", "America/Nonexistent"];
+
+for (const timeZone of unknown) {
+  assert.throws(
+    RangeError,
+    () => instance.toString({ timeZone }),
+    `${timeZone} is not an available named time zone`
+  );
+
+  assert.throws(
+    RangeError,
+    () => instance.toString({ timeZone: `1970-01-01T00:00+01:00[${timeZone}]` }),
+    `${timeZone} in a time zone annotation is not an available named time zone, and the offset is not a fallback`
+  );
+}
diff --git a/test/built-ins/Temporal/Instant/prototype/toZonedDateTimeISO/timezone-string-unknown.js b/test/built-ins/Temporal/Instant/prototype/toZonedDateTimeISO/timezone-string-unknown.js
new file mode 100644
index 00000000000..cd717594a07
--- /dev/null
+++ b/test/built-ins/Temporal/Instant/prototype/toZonedDateTimeISO/timezone-string-unknown.js
@@ -0,0 +1,26 @@
+// Copyright (C) 2026 Rudolph Gottesheim. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.instant.prototype.tozoneddatetimeiso
+description: Unrecognized time zone identifiers are not valid input for a time zone
+features: [Temporal]
+---*/
+
+const instance = new Temporal.Instant(0n);
+
+const unknown = ["Mars/Olympus_Mons", "America/Nonexistent"];
+
+for (const timeZone of unknown) {
+  assert.throws(
+    RangeError,
+    () => instance.toZonedDateTimeISO(timeZone),
+    `${timeZone} is not an available named time zone`
+  );
+
+  assert.throws(
+    RangeError,
+    () => instance.toZonedDateTimeISO(`1970-01-01T00:00+01:00[${timeZone}]`),
+    `${timeZone} in a time zone annotation is not an available named time zone, and the offset is not a fallback`
+  );
+}