Peniocereus is a genus of vining cacti, comprising about 18 species, found from the southwestern United States and Mexico.

Its name comes from the prefix penio- (from the Latin penis, meaning ‘tail’) and Cereus, the large genus from which it was split.

In 2005 a molecular phylogenetic study of the genus supported this split and showed that Peniocereus is not monophyletic.

[2] The 2005 molecular study showed that P. serpentinus is in Echinocereeae along with subgenus Peniocereus, but suggests resurrecting the monotopic Nyctocereus as it is sister to Bergerocactus.

[2] Molecular phylogeny and morphological evidence suggests this subgenus is more closely related to Acanthocereus.