Lacus Autumni (Latin for "Lake of Autumn") is a region of lunar mare that lies near the western limb of the Moon. Along this side of the lunar surface is a huge impact basin centered on the Mare Orientale. Two concentric mountain rings surround the Orientale mare, the inner ring being named Montes Rook and an outer ring called the Montes Cordillera. Lacus Autumni lies in the northeastern quadrant of the gap between these two mountain rings. Unfortunately this section of the lunar surface is difficult to observe directly from the Earth.
This mare has selenographic coordinates of 9.9° S, 83.9° W, and it lies within a diameter of 183 km. (Compare to the Mare Orientale, which has a diameter of 327 km.) This mare runs from the southeast to the northwest, reaching a maximum width of 90-100 km. Only the central and southeastern sections of the lake have regions of relatively low albedo material typical of a lunar mare. The remainder matches the albedo of the surrounding terrain.