Megaselia nantucketensis new species (Phoridae) is described from Nantucket Island, Massachusetts, USA. The single specimen was reared from a leaf gall on black oak (Fagaceae: Quercus velutina Lam.) caused by a species of Macrodiplosis Kieffer (Cecidomyiidae). The larva of this univoltine fly is presumed to be a predator on larvae of the gall midge.