|Feature on||The Moon|
|Feature type||Ringed plain|
|Lunar coordinates||9° S, 2° W|
|Dimensions||Diameter 153km; depth 2,400m|
A large and prominent ringed plain near the centre of the visible disc of the Moon, the northernmost of a chain of signifcant features running down the eastern side of the Mare Nubium. Ptolemaeus is a circular feature a little over 150 km from edge to edge. Its southern wall has been obliterated by a later event, which formed the crater Alphonsus, which (with a diameter of 118 km) is only a little smaller than Ptolemaeus itself.