Epitaxially grown ultrathin organic semiconductors on graphene show great promise as highly efficient phototransistors. The devices exhibit a strong photoresponse down to the limit of a monolayer organic crystal, with a photoresponsivity higher than 10(4) A W-1 and a photoconductive gain over 10(8). The excellent performance is attributed to the high quality of the organic crystal and interface, a unique feature of van der Waals epitaxy.