Francisella halioticida is a marine bacterium originally described as the causative agent of mass mortality among giant abalone Haliotis gigantea. Recent field studies in Canada and Japan have suggested that this bacterium is also the cause of adductor muscle lesions and high mortality of Yesso scallops Patinopecten yessoensis, although a causal relationship has not been established. In the present study, the pathogenicity of F. halioticida in Yesso scallops was assessed in both Canada and Japan using bacteria isolated from diseased Yesso scallops in each respective country. Independent laboratory experiments revealed that scallops challenged with F. halioticida via bath exposure resulted in high mortality and histological lesions characterized by massive haemocyte infiltration. The presence of F. halioticida was confirmed using PCR, and F. halioticida was re-isolated from a portion of dead and surviving specimens. These results fulfill Koch's classic criteria for establishing disease causation and provide conclusive evidence that F. halioticida causes adductor muscle lesions and high mortality in Yesso scallops.