Night Girl is a fictional character in the DC Universe, a member of the Legion of Substitute Heroes in the 30th century. Her real name is Lydda Jath and her home planet is Kathoon, a world which is in perpetual darkness, heated by an internal power source. Lydda has super strength, given to her by her scientist father, but only at night or in deep shadow.

She first appeared in Adventure Comics#306. In pre-Zero Hour continuity, she unsuccessfully tried out for the Legion of Super-Heroes who rejected her as a power that only worked in darkness was impractical. She then went on to be a founding member of the Substitute Legion. She had a crush on Cosmic Boy and that was her motivation for leaving Kathoon and trying out for the Legion in the first place. Years later she and Cosmic Boy started a relationship and eventually married. In post-Zero Hour continuity she only made two very brief appearances, as she was rejected at a try out once again and joined the Substitute Legion once more, she was seen in one panel in a later issue as she and Polar Boy talked about how the Substitute Legion had a long way to go until they were ready to take on a mission but thereafter no more was seen of the post-Zero Hour Night Girl.