Triplet transfer across a surface-anchored metal-organic-framework heterojunction is demonstrated by the observation of triplet-triplet annihilation photon upconversion in a sensitizer-emitter heterostructure. Upconversion thresholds under 1 mW cm(-2) are achieved. In the broader context, the double-electron-exchange mechanism of triplet transfer indicates that the heterojunction quality is sufficient for electrons to move between layers in this solution-processed crystalline heterostructure.