T. Mayer, or Tobias Mayer, is a lunar impact crater that is located at the western end of the Montes Carpatus mountain range along the southern edge of Mare Imbrium. It was named after German astronomer Tobias Mayer. To the west is the Oceanus Procellarum, and to the south is Mare Insularum. The crater is located a couple of hundred kilometers to the northwest of the prominent crater Copernicus.