The most notable feature of this crater is the disproportionately large central peak, which forms a rounded rise that occupies almost the entire crater floor—one-third the crater diameter—and rises to a height of 2.0 km. There is evidence that this peak has been enlarged through volcanic eruptions, and there appears to be an eroded vent at its summit. The rim has fine terraces that slope down to near the edge of the central rise along the south and east sides. This outer wall is nearly round, with slight protrusions on the north and west sides.
The outer rim is joined to the southwest rim of Alphonsus by a rise in the surface. An arc of craterous depressions from the south rim of Alphonsus curves to the west, dividing Alpetragius from Arzachel crater. To the west-northwest is the crater-like outline of Alpetragius X, now flooded by the mare and overlain across the east by ejecta from Alpetragius.
Alpetragius is similar in appearance to Behaim crater on the far side, although Behaim is larger.
Alpetragius is a Latinization of the name of Nur ed-Din al Betrugi. Like many of the craters on the Moon's near side, Alpetragius was named by Giovanni Riccioli, whose 1651 nomenclature system has become standardized. Earlier lunar cartographers had given the feature different names: Michael van Langren's 1645 map calls it "Mazzarinii"