The present paper describes a transitivity distinction that is attested in some Tibeto-Burman (TB) languages of the West Himalayish (WH) subgroup. The relevant distinction is encoded by a set of dedicated markers that occur between verb stems and inflectional endings and group verb into transitivity classes. The paper first offers a synchronic description of transitivity classes in the WH language Bunan, discussing their formal realization and functional motivation. Subsequently, the relevant transitivity classes are discussed from a historical-comparative perspective. It is argued that the transitivity distinction developed when an object agreement marker was reanalyzed as a marker of transitive verbs. The paper thus offers new perspectives on transitivity in TB from both a synchronic and a diacronic point of view, and adduces evidence for a hitherto underscribed reanalysis from “object agreement marker” > “marker of transitive verbs”.