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Abstract Ocean warming caused by global climate change is driving range expansions and shifts in marine species. The lady crab Ovalipes ocellatus (Herbst, 1799) is generally found south of Cape Cod, Massachusetts, USA with a disjunct population in the southern Gulf of St. Lawrence, Canada, but absent in the Gulf of Maine and Bay of Fundy. Here we present trawl survey data, recent crowd-sourced observations, and temperature data that suggest a range expansion of O. ocellatus north of Cape Cod into the Gulf of Maine and Bay of Fundy after a marine heat wave in 2012. Crowd-sourced observations of lady crabs increased in the Gulf of Maine at the same time that abundances surged after 2000. In the Gulf of Maine, O. ocellatus was found as far north as Freeport, Maine (43°48′17.136″N, 70°6′30.9594″W) and in the Bay of Fundy as far north as Alma, New Brunswick, Canada (45°36′ 13.6794″N, 64°56′29.184″W). We also extend the southern limit of O. ocellatus to St. Augustine, Florida, USA (29°42′9.432″N, 81°13′56.028″ W). The recent observations of O. ocellatus in the northwestern Atlantic and higher abundances combined with continued warming in this area may signal a permanent expansion of this species. If so, a key goal for ecologists and managers will be to understand the effects of O. ocellatus on food webs and fisheries in the Gulf of Maine and Bay of Fundy.