The results from an experimental study of a model diffuser augmented tidal stream turbine are presented with a particular focus on the impact of the diffuser upon the turbine's performance in yawed flows. This study is the first to examine yaw effects with quantification of blockage corrections and is the first study of the wake recovery characteristics of such devices. The device was designed using an innovative optimisation procedure resulting in a diffuser that was able to maintain the turbine׳s performance to yaw angles of up to ±30°. It is shown that the diffuser׳s performance is strongly influenced by its length to diameter ratio and by the jet flow that develops through the turbine׳s tip gap. Although the performance characteristics of an individual turbine can be significantly improved by diffuser augmentation under yawed flow, a wake recovery rate that is less than half that of a bare rotor raises doubts about their suitability for array deployment.