Tidal stream energy has great potential to reduce the greenhouse gas emissions of electricity supply, but development has been slow, in part due to high costs and challenging engineering required to operate in the marine environment. Floating support structures may be able to help reduce these costs. A laboratory study comparing the performance of a turbine mounted on fixed and floating support structures was used to investigate this potential. Realistic flow and wave cases were established from two installation sites. The turbine was found to generate greater power when mounted on a floating structure across all wave and flow cases, and the support structure experienced less strain. In both cases, the ability of the floating structure to move in response to wave action is believed to be the key factor. Floating support structures are not without challenges, but may offer a route to lower cost tidal stream turbine installations.