The broad metabolic capacity of Shewanella species allows them to colonise a wide range of aquatic niches. Their ability to convert sulphur compounds, including tetrathionate, has been reported. This may seem surprising, as this ability has long been considered restricted to human gut pathogens such as Enterobacteria and tetrathionate is considered unstable in the external environment. The molecular basis of Shewanella growth on tetrathionate had never been analysed. By combining the construction and metabolic characterisation of deletion mutants and complementary biochemical analyses, we determined that Shewanella sp. ANA‐3 uses two enzymes, homologous to the tetrathionate reductase Ttr and the polysulphide reductase Psr, respectively, to respire tetrathionate. This study provides the first evidence that a Psr homologue can catalyse this reaction. Neither the octahaem tetrathionate reductase OTR nor the thiosulfate dehydrogenase Tsd, which were also examined, participate in this respiration. Although reduction of tetrathionate is the known physiological function of the Ttr, this work represents the first rigorous establishment of this role in an environmental microorganism. Based on the relatively high redox potential of the tetrathionate/thiosulphate redox couple and the wide distribution of ttr genes, we discuss the hypothesis of widespread distribution of Ttr‐based tetrathionate respiration in the external environment.