Haplogroup S also known as S-B254 is a human Y-DNA haplogroup, defined by the SNPs B254 and Z33355.^[1] Until 2016, it was known as Haplogroup S1.

S-M254 is the most common haplogroup among males in the Papua New Guinea Highlands. It is also relatively common in some parts of Oceania, Wallacea and among indigenous Australians.^{[citation needed]}

In a major correction to the phylogenetic tree during 2016, Haplogroup S-M230 was "demoted" from the position of basal clade S*, becoming S1a1b, while S-B254 was "promoted" in its place to the level of S*.^{[citation needed]}

It has two primary subclades: S1 (B255) and S2 (P378).^[1] The existence of a third subclade, defined by the SNP, P336 has not been confirmed.