The biosynthetic route for homocysteine, intermediate in methionine biosynthesis, is unknown in some methanogenic archaea because homologues of the canonical required genes cannot be identified. Here we demonstrate that Methanocaldococcus jannaschii can biosynthesize homocysteine from aspartate semialdehyde and hydrogen sulfide. Additionally, we confirm the genes involved in this new pathway in Methanosarcina acetivorans. A possible series of reactions in which a thioaldehyde is formed and then reduced to a thiol are proposed. This represents a novel route for the biosynthesis of homocysteine and exemplifies unique aspects of sulfur chemistry occurring in prebiotic environments and in early life forms.