diff --git a/crates/jett_comptime/src/interpreter.rs b/crates/jett_comptime/src/interpreter.rs
index 33ca4ac..88b01a9 100644
--- a/crates/jett_comptime/src/interpreter.rs
+++ b/crates/jett_comptime/src/interpreter.rs
@@ -8786,7 +8786,12 @@ impl Interpreter {
                         } else {
                             let mut result: i64 = 1;
                             for i in 2..=*n {
-                                result = result.saturating_mul(i);
+                                let Some(next_result) = result.checked_mul(i) else {
+                                    return Some(Err(format!(
+                                        "math.factorial: integer overflow: {result} * {i}"
+                                    )));
+                                };
+                                result = next_result;
                             }
                             Some(Ok(Value::Int64(result)))
                         }
diff --git a/crates/jett_driver/tests/fixture_suite.rs b/crates/jett_driver/tests/fixture_suite.rs
index 354e5e4..7e5bf99 100644
--- a/crates/jett_driver/tests/fixture_suite.rs
+++ b/crates/jett_driver/tests/fixture_suite.rs
@@ -426,6 +426,14 @@ fn math_sum_reports_overflow() {
     );
 }
 
+#[test]
+fn math_factorial_reports_overflow() {
+    assert_runtime_fail(
+        "math_factorial_overflow.jett",
+        "runtime error: math.factorial: integer overflow: 2432902008176640000 * 21",
+    );
+}
+
 #[test]
 fn math_clamp_rejects_invalid_bounds() {
     let cases = [
diff --git a/tests/runtime_fail/math_factorial_overflow.jett b/tests/runtime_fail/math_factorial_overflow.jett
new file mode 100644
index 0000000..ccfecfa
--- /dev/null
+++ b/tests/runtime_fail/math_factorial_overflow.jett
@@ -0,0 +1,3 @@
+function main() returns nothing:
+    int64 result = math.factorial(21)
+    return nothing