The present paper looks into the effects of spinodal decomposition (SD) on the corrosion resistance and cavitation erosion behaviour of the duplex stainless steel (DSS) 7MoPLUS. SD takes place at 500 °C in the ferrite phase at the early stage of thermal ageing, with an attendant hardness increase. Nevertheless, thermal ageing at this temperature for up to 2 days nearly does not reduce pitting potential (in 3.5% NaCl). Surface integrity, however, is degraded by the appearance of noticeable pits after just a few hours of exposure to this temperature. Ageing for 1 week at 500 °C does not seem to alter the corrosion morphology of the ferrite phase in 1 M HCl. On the other hand, SD does bring about changes to the nature of passive film, as demonstrated by open-circuit potential measurements in 3.5% NaCl. The cavitation erosion (CE) resistance of 7MoPLUS is slightly enhanced by short-term SD treatment (for 5 h), as propagation of CE damage emanating from deformation twins in the ferrite phase is made less efficient. It is suggested that short-term SD treatment is a feasible strengthening method for DSSs.