Right whales (Balaenidae) are the most distinctive family of extant baleen whales, thanks to their highly arched rostrum, tall lips and robust body shape. They are also the oldest, originating as much as 20 million years ago (Ma). Nevertheless, their fossil record is patchy and frequently understudied, obscuring their evolution. Here, we describe a new stem balaenid, Antwerpibalaena liberatlas , from northern Belgium, adding to the rich but historically problematic baleen whale assemblage of the Pliocene North Sea. Within right whales, Antwerpibalaena forms a clade with two previously described extinct genera, Balaenella and Balaenula . The holotype preserves much of the postcranial skeleton, and informs the emergence of typical balaenid traits like fused neck vertebrae and paddle-shaped flippers. Its size is intermediate between that of extant right whales and most of their extinct forebears revealing a more complex pattern of balaenid size evolution than previously thought. ...