Yersinia ruckeri is the causative agent of enteric redmouth disease or yersiniosis, which affects mainly salmonids. This microorganism has been consistently causing economic losses in the aquaculture industry since its first description; but the early development of a vaccine allowed a relative control of the disease. This might be the reason why the specific pathogenicity mechanisms of this bacterium remained elusive until recently, when the results obtained with traditional microbiology have been complemented with those provided by molecular biology. The data obtained by using these novel techniques, which are the main subject of this review, have started to shed light on the virulence of this pathogen. Thus, iron acquisition by the siderophore ruckerbactin, proteolytic and haemolytic activities, and resistance to immune mechanisms, were proved to be involved in the virulence of this bacterium. Additionally, these data will, in the long term, help clarify the controversial taxonomic status of this microorganism and allow the development of novel ways to prevent outbreaks, which is particularly interesting nowadays, given that commercial vaccines seem to be ineffective against some new isolates.