diff --git a/Userland/Libraries/LibJS/Runtime/ErrorTypes.h b/Userland/Libraries/LibJS/Runtime/ErrorTypes.h
index 8f2929293a9..1fd4a9c879b 100644
--- a/Userland/Libraries/LibJS/Runtime/ErrorTypes.h
+++ b/Userland/Libraries/LibJS/Runtime/ErrorTypes.h
@@ -215,6 +215,7 @@
     M(TemporalInvalidDurationPropertyValueNonIntegral, "Invalid value for duration property '{}': must be an integer, got {}")          \
     M(TemporalInvalidDurationPropertyValueNonZero, "Invalid value for duration property '{}': must be zero, got {}")                    \
     M(TemporalInvalidEpochNanoseconds, "Invalid epoch nanoseconds value, must be in range -86400 * 10^17 to 86400 * 10^17")             \
+    M(TemporalInvalidInstantString, "Invalid instant string '{}'")                                                                      \
     M(TemporalInvalidISODate, "Invalid ISO date")                                                                                       \
     M(TemporalInvalidMonthCode, "Invalid month code")                                                                                   \
     M(TemporalInvalidMonthDayString, "Invalid month day string '{}'")                                                                   \
diff --git a/Userland/Libraries/LibJS/Runtime/Temporal/AbstractOperations.cpp b/Userland/Libraries/LibJS/Runtime/Temporal/AbstractOperations.cpp
index 482718ec3b4..1ae9d5fe00e 100644
--- a/Userland/Libraries/LibJS/Runtime/Temporal/AbstractOperations.cpp
+++ b/Userland/Libraries/LibJS/Runtime/Temporal/AbstractOperations.cpp
@@ -1161,14 +1161,19 @@ ThrowCompletionOr<ISODateTime> parse_iso_date_time(GlobalObject& global_object,
 // 13.35 ParseTemporalInstantString ( isoString ), https://tc39.es/proposal-temporal/#sec-temporal-parsetemporalinstantstring
 ThrowCompletionOr<TemporalInstant> parse_temporal_instant_string(GlobalObject& global_object, String const& iso_string)
 {
+    auto& vm = global_object.vm();
+
     // 1. Assert: Type(isoString) is String.
 
     // 2. If isoString does not satisfy the syntax of a TemporalInstantString (see 13.33), then
-    // a. Throw a RangeError exception.
-    // TODO
+    auto parse_result = parse_iso8601(Production::TemporalInstantString, iso_string);
+    if (!parse_result.has_value()) {
+        // a. Throw a RangeError exception.
+        return vm.throw_completion<RangeError>(global_object, ErrorType::TemporalInvalidInstantString, iso_string);
+    }
 
     // 3. Let result be ! ParseISODateTime(isoString).
-    auto result = MUST(parse_iso_date_time(global_object, {}));
+    auto result = MUST(parse_iso_date_time(global_object, *parse_result));
 
     // 4. Let timeZoneResult be ? ParseTemporalTimeZoneString(isoString).
     auto time_zone_result = TRY(parse_temporal_time_zone_string(global_object, iso_string));
diff --git a/Userland/Libraries/LibJS/Runtime/Temporal/ISO8601.cpp b/Userland/Libraries/LibJS/Runtime/Temporal/ISO8601.cpp
index 697174f727c..eec40e1e3e1 100644
--- a/Userland/Libraries/LibJS/Runtime/Temporal/ISO8601.cpp
+++ b/Userland/Libraries/LibJS/Runtime/Temporal/ISO8601.cpp
@@ -746,6 +746,27 @@ bool ISO8601Parser::parse_calendar_date_time()
     return true;
 }
 
+// https://tc39.es/proposal-temporal/#prod-TemporalInstantString
+bool ISO8601Parser::parse_temporal_instant_string()
+{
+    // TemporalInstantString :
+    //     Date TimeZoneOffsetRequired
+    //     Date DateTimeSeparator TimeSpec TimeZoneOffsetRequired
+    StateTransaction transaction { *this };
+    if (!parse_date())
+        return false;
+    if (!parse_time_zone_offset_required()) {
+        if (!parse_date_time_separator())
+            return false;
+        if (!parse_time_spec())
+            return false;
+        if (!parse_time_zone_offset_required())
+            return false;
+    }
+    transaction.commit();
+    return true;
+}
+
 // https://tc39.es/proposal-temporal/#prod-TemporalDateString
 bool ISO8601Parser::parse_temporal_date_string()
 {
@@ -874,6 +895,7 @@ bool ISO8601Parser::parse_temporal_relative_to_string()
 }
 
 #define JS_ENUMERATE_ISO8601_PRODUCTION_PARSERS                                        \
+    __JS_ENUMERATE(TemporalInstantString, parse_temporal_instant_string)               \
     __JS_ENUMERATE(TemporalDateString, parse_temporal_date_string)                     \
     __JS_ENUMERATE(TemporalDateTimeString, parse_temporal_date_time_string)            \
     __JS_ENUMERATE(TemporalMonthDayString, parse_temporal_month_day_string)            \
diff --git a/Userland/Libraries/LibJS/Runtime/Temporal/ISO8601.h b/Userland/Libraries/LibJS/Runtime/Temporal/ISO8601.h
index bb019f168ca..cf238aa235d 100644
--- a/Userland/Libraries/LibJS/Runtime/Temporal/ISO8601.h
+++ b/Userland/Libraries/LibJS/Runtime/Temporal/ISO8601.h
@@ -33,6 +33,7 @@ struct ParseResult {
 };
 
 enum class Production {
+    TemporalInstantString,
     TemporalDateString,
     TemporalDateTimeString,
     TemporalMonthDayString,
@@ -106,6 +107,7 @@ public:
     [[nodiscard]] bool parse_time_spec_separator();
     [[nodiscard]] bool parse_date_time();
     [[nodiscard]] bool parse_calendar_date_time();
+    [[nodiscard]] bool parse_temporal_instant_string();
     [[nodiscard]] bool parse_temporal_date_string();
     [[nodiscard]] bool parse_temporal_date_time_string();
     [[nodiscard]] bool parse_temporal_month_day_string();
diff --git a/Userland/Libraries/LibJS/Tests/builtins/Temporal/Instant/Instant.from.js b/Userland/Libraries/LibJS/Tests/builtins/Temporal/Instant/Instant.from.js
index e182a916203..72c67e68bc6 100644
--- a/Userland/Libraries/LibJS/Tests/builtins/Temporal/Instant/Instant.from.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Temporal/Instant/Instant.from.js
@@ -14,10 +14,17 @@ describe("correct behavior", () => {
         expect(Temporal.Instant.from(zonedDateTime).epochNanoseconds).toBe(123n);
     });
 
-    // Un-skip once ParseISODateTime & ParseTemporalTimeZoneString are implemented
-    test.skip("Instant string argument", () => {
+    test("Instant string argument", () => {
         expect(Temporal.Instant.from("1975-02-02T14:25:36.123456789Z").epochNanoseconds).toBe(
             160583136123456789n
         );
     });
 });
+
+describe("errors", () => {
+    test("invalid instant string", () => {
+        expect(() => {
+            Temporal.Instant.from("foo");
+        }).toThrowWithMessage(RangeError, "Invalid instant string 'foo'");
+    });
+});