diff --git a/rmp-serde/src/encode.rs b/rmp-serde/src/encode.rs
index f42b20b7..d745b96f 100644
--- a/rmp-serde/src/encode.rs
+++ b/rmp-serde/src/encode.rs
@@ -604,7 +604,12 @@ where
     }
 
     fn serialize_i128(self, v: i128) -> Result<Self::Ok, Self::Error> {
-        self.serialize_bytes(&v.to_be_bytes())
+        if let Ok(v_i64) = i64::try_from(v) {
+            encode::write_sint(&mut self.wr, v_i64)?;
+        } else {
+            self.serialize_bytes(&v.to_be_bytes())?;
+        }
+        Ok(())
     }
 
     fn serialize_u8(self, v: u8) -> Result<Self::Ok, Self::Error> {
@@ -625,7 +630,12 @@ where
     }
 
     fn serialize_u128(self, v: u128) -> Result<Self::Ok, Self::Error> {
-        self.serialize_bytes(&v.to_be_bytes())
+        if let Ok(v_u64) = u64::try_from(v) {
+            encode::write_uint(&mut self.wr, v_u64)?;
+        } else {
+            self.serialize_bytes(&v.to_be_bytes())?;
+        }
+        Ok(())
     }
 
     fn serialize_f32(self, v: f32) -> Result<Self::Ok, Self::Error> {
diff --git a/rmp-serde/tests/round.rs b/rmp-serde/tests/round.rs
index 120c2e60..97d10f59 100644
--- a/rmp-serde/tests/round.rs
+++ b/rmp-serde/tests/round.rs
@@ -666,6 +666,27 @@ fn roundtrip_some_failures() {
     assert_roundtrips(Some(None::<()>));
 }
 
+#[test]
+fn roundtrip_i128_and_u128() {
+    assert_roundtrips(i8::MIN as i128);
+    assert_roundtrips(i16::MIN as i128);
+    assert_roundtrips(i32::MIN as i128);
+    assert_roundtrips(i64::MIN as i128);
+    assert_roundtrips(i128::MIN);
+
+    assert_roundtrips(i8::MAX as i128);
+    assert_roundtrips(i16::MAX as i128);
+    assert_roundtrips(i32::MAX as i128);
+    assert_roundtrips(i64::MAX as i128);
+    assert_roundtrips(i128::MAX);
+
+    assert_roundtrips(u8::MAX as u128);
+    assert_roundtrips(u16::MAX as u128);
+    assert_roundtrips(u32::MAX as u128);
+    assert_roundtrips(u64::MAX as u128);
+    assert_roundtrips(u128::MAX);
+}
+
 #[cfg(test)]
 #[track_caller]
 fn assert_roundtrips<T: PartialEq + std::fmt::Debug + Serialize + for<'a> Deserialize<'a>>(val: T) {