Shiga toxin‐producing Escherichia coli are a known cause of serious illnesses, and a major public health concern. The coastal environment is the receptacle of agricultural and urban wastewater effluents, thus STEC can be present. In the current study their occurrence and prevalence in mussels (Mytilus galloprovincialis) cultured in the southern Rias of Galician was assessed. This region is interesting due to its economic importance as Galicia produces the 98% of the mussels cultured in Spain, being this country the third world producer. From June 2012 until September 2013, 241 mussel samples were collected, being 112 positive for eae and 17 also positive for stx1,2. An increase was observed in the winter months for both genes, which seemed correlated with a generalized decrease in water salinity. Serotype O157:H7 was not detected, but one isolate resulted resistant to six antibiotics, denoting the presence of multidrug resistant bacteria in this área. Practical applications A critical step for adequate risk assessments is the evaluation of potential threats. In this sense, very few microbiological parameters are analyzed in bivalve molluscs, being many human pathogens not assayed. Shiga toxin‐producing Escherichia coli was detected for the first time in mussels harvested in Galicia, a region of critical importance worldwide for the mussel industry. This study highlights that additional studies to better characterize the microbial populations of this important region are still needed, and furthermore, that measures to control microbial pathogens, such as depuration, must not be taken lightly.