diff --git a/tests/serde/iso8601.rs b/tests/serde/iso8601.rs
index 471a093ec..096d09caa 100644
--- a/tests/serde/iso8601.rs
+++ b/tests/serde/iso8601.rs
@@ -89,3 +89,25 @@ fn deserialize_error() {
         "the 'year' component could not be parsed",
     );
 }
+
+#[test]
+fn issue_674_leap_second_support() {
+    serde_test::assert_de_tokens::<Test>(
+        &Test {
+            dt: datetime!(2016-12-31 23:59:59.999999999 UTC),
+            option_dt: Some(datetime!(2016-12-31 23:59:59.999999999 UTC)),
+        },
+        &[
+            Token::Struct {
+                name: "Test",
+                len: 2,
+            },
+            Token::Str("dt"),
+            Token::BorrowedStr("2016-12-31T23:59:60.000Z"),
+            Token::Str("option_dt"),
+            Token::Some,
+            Token::BorrowedStr("2016-12-31T23:59:60.000Z"),
+            Token::StructEnd,
+        ],
+    );
+}
diff --git a/time/src/parsing/parsable.rs b/time/src/parsing/parsable.rs
index 36f663b1f..1679f6368 100644
--- a/time/src/parsing/parsable.rs
+++ b/time/src/parsing/parsable.rs
@@ -739,6 +739,8 @@ impl<const CONFIG: EncodedConfig> sealed::Sealed for Iso8601<CONFIG> {
         let mut offset_is_present = false;
         let mut first_error = None;
 
+        parsed.leap_second_allowed = true;
+
         match Self::parse_date(parsed, &mut extended_kind)(input) {
             Ok(new_input) => {
                 input = new_input;