Penion maximus is a species of very large predatory sea snail or whelk, commonly known as giant whelk or great whelk, a marine gastropod mollusc in the family Austrosiphonidae.

[2][3] The species could be confused with the sympatric species P. mandarinus, however P. mandarinus is typically smaller and has a smoother shell with a shorter siphonal canal.

The range of the species extends from waters off Tasmania and Victoria to lower Queensland.

[2][3] Penion maximus is closely related to another Australian species P.

[3][4] The species have overlapping geographic ranges (sympatry) and may have evolved from a common ancestor via niche differentiation based on prey size and water depth.