The aim of this research was to integrate sustainable development and design-thinking-based product design in order that the product design then contributes to society's transition to a sustainable future. This is an important pursuit since product lifecycles are a major cause of society's current sustainability challenges. To address this, many authors argue for integrating sustainable development into existing design processes rather than developing stand-alone tools and methods. Through action research with a design consultancy who wanted to start working with sustainable product design, we iterated between three stages: understanding needs, designing action, and trying out the action. The first stage comprised document analysis, focus-group style workshops, a survey and interviews. When designing the actions (enhancements to their design-thinking-based process), we drew on literature on sustainable product design, decision-making for sustainability, and organisational learning and change for sustainability. We also drew on our research partners' practical experience. The enhanced process was tried out and further developed through feedback, student testing and co-development meetings. The result is an enhanced process where project teams (i) use the outcomes from the inspiration phase of the existing process to choose sustainable design strategies that are relevant for their particular project. Once the teams have chosen which strategies to work with, for example, design for remanufacture, we suggest that they (ii) use the strategies to develop ideation foci/questions that help them explore the design space. The third enhancement is for teams to (iii) compare concepts with respect to sustainability as part of their concept comparison and evaluation.