Posidonius est un cratère à fond fracturé de 95 Km et profond de 2300 m situé sur le bord oriental de la mer de la sérénité (Mare Serenitatis) et au sud du Lac des Songes (Lacus Somniorum). Le cratère Chacornac (51 Km, 1450 m) est attaché au mur Sud-est de Posidonius. La muraille de Posidinius est peu élevée et échancrée, particulièrement sur le bord occidental. L'intérieur a été inondé par un écoulement de lave dans le passé. On peut encore observer les remparts intacts du cratère au sud et à l'est du  cratère. Il y a une plus petite et semi-circulaire muraille d'un cratère concentrique et inondé à l’intérieur, excentrée vers le bord oriental. Le plancher est accidenté et lacéré par un système de rainures appelées Rimae Posidinius. Le magma intérieur en s'élevant, poussa sous Posidonius, surélevant et fracturant son sol. La lave surgit, remplit partiellement le cratère et créa les rainures sinueuses. Le petit demi-cercle montagneux près du centre peut être les sommets d'un grand complexe de pics centraux enterrés dans la lave. Le mur Nord-est est interrompu par le cratère plus petit Posidonius B (14 Km). Dans Posidonius, juste à l'ouest du centre on trouve Posidinius A.

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Posidonius is a lunar impact crater that is located on the western edge of Mare Serenitatis, to the south of Lacus Somniorum. The Chacornac crater is attached to the southeast rim, and to the north is Daniell crater.
The rim of Posidinius is shallow and obscured, especially on the western edge, and the interior has been overlaid by a lava flow in the past. The crater ramparts can still be observed to the south and east of the crater rim, and to a lesser degree to the north.
There is a smaller, semi-circular rim of a concentric, flooded crater within the main rim, offset toward the eastern edge. There is no central peak, but the floor is hilly and laced with a rille system named the Rimae Posidinius. The floor is also slightly buldged due to the past lava uplift, which also likely produced the complex of rilles. The northeast rim is interrupted by the smaller crater 'Posidonius B'. Within the crater rim, offset just to the west of center is another smaller crater 'Posidinius A'.
On the Mare Serenitatis surface near Posidonius crater is a notable system of wrinkle-ridges that parallel the nearby shore. These are designated the Dorsa Smirnov. At the peak of these ridges is a small craterlet with a diameter of 2 km. This craterlet is surrounded by a patch of high albedo material, and is an example of a lunar bright spot. This peak was formerly designated Posidonius Gamma (γ).
The Posidonius Gamma feature was first observed by the lunar cartographer Julius Schmidt in 1857. He noted the similarity to the bright patch surrounding Linné crater.