Planaxis is one of six extant genera of marine prosobranch gastropods, place within a small, monophyletic group of marine taxa in the Planaxidae family (Houbrick 1987). The family Planaxidae is placed further within the superfamily Cerithiacea; a large superfamily of caenogastropods that includes 17 recent families, approximately 200 extant generas and 1100 extant species (Houbrick 1987; Strong et al. 2011).   Members of Cerithiacea are found over a wide geographical range in the tropics, sub-tropics and warm temperate regions, occuying a wide diversity of aquatic environments (Strong et al. 2011).

Planaxidae is not as species-rich when compared to other members of the Cerithiacea such as the related families of Cerithiidae, Thiarridae and Potamididae (Houbrick 1987; Strong et al. 2011). The family Planaxidae, which consists of 6 extant and 2 extinct genera, is currently comprised of 20 separate and valid species (Houbrick 1987; Strong et al. 2011). The genera Planaxis were among the earliest representatives of the Planaxidae family to be uncovered from the fossil records and P. sulcatus is one of three species of Planaxids discovered in the fossil records to be  still extant today (Houbrick 1987). A derived phylogenetic tree based on morphologic features and molecular genetics evidences modified from Houbrick (1987) and Strong (2011) is illustrated below (Houbrick 1987; Strong et al. 2011).