AbstractThis is the first study to document quantitatively that Gulf of Maine (GOM) Winter Flounder Pseudopleuronectes americanus spawn offshore. Three sites (southern Jeffreys Ledge, Bigear [an area southwest of Tillies Bank], and a portion of Stellwagen Bank National Marine Sanctuary) were sampled during spring 2016 to determine whether Winter Flounder are spawning in non‐coastal, deep waters in the southern GOM. In total, 1,384 Winter Flounder were caught by trawl, measured, sexed, and assessed for reproductive stage during the peak spawning season (March–May). These fish showed clear signs that spawning was occurring either at or very near to all three sites surveyed. In all sites, a shift from prespawn to postspawn females occurred. Running ripe females were caught at both Jeffreys Ledge and Stellwagen Bank on multiple occasions, and ripe and recently spawned females were caught at Bigear. Given that these sites are over 15 km from the coast, it is not energetically likely that fish in such advanced reproductive condition are migrating to shore to spawn. Essential fish habitat (EFH) for GOM Winter Flounder is categorized as waters with depths of 5 m or less. The present study clearly shows that this depth range is incomplete and thus inaccurate for southern GOM Winter Flounder. Although it is irrefutable that GOM estuaries and shallow bays are important as nursery grounds for juvenile Winter Flounder and as postspawn feeding grounds for adults, the current GOM EFH designation warrants reclassification.