Neptune doesn't have a surface in the way we think of it, so it doesn't have a single "highest temperature". Instead, it has different temperatures at different depths and atmospheric levels.

However, we can talk about the hottest part of its atmosphere.

The hottest part of Neptune's atmosphere is at the top of the clouds, which is about -214°C (-353°F).

This may seem incredibly cold, and it is! But it's actually warmer than the deeper layers of its atmosphere. The reason for this is that Neptune receives very little sunlight, and its internal heat source is what drives the warmer temperatures in its upper atmosphere.