Introduction The latest climate reports underscore the importance of climate change mitigation behaviours in reducing greenhouse gas emissions and achieving the climate goals. There is a growing demand among the general population for guidance on sustainable behaviour change. Previous research has indicated that behavioural interventions could play a significant role in facilitating this type of behaviour change. However, the effectiveness of such approaches has yet to be rigorously tested under scientific conditions. This project aims to assess whether a novel, intervention “Sustainable Choices” effectively promotes climate change mitigation behaviours when compared to a control condition. Methods Voluntary participants will be recruited from the general population in Sweden and randomized to either “Sustainable Choices”, an online, behavioural intervention targeted at Climate Change Mitigation Behaviours (CCMBs), or a waitlist, both 5 weeks of duration. The target sample size is N=680. The primary outcome of the study will be pre- to post-intervention difference in climate change mitigation behaviours, measured with the Climate Mitigation Behaviour Scale (CLIMBS). Secondary outcomes will include acceptability measures and effects on psychological well-being. Naturalistic follow-up assessments will be administered at 1-, 3- and 6-months after the post-measurement. Discussion The increasing awareness of the link between climate change and individual sustainable lifestyle choices has generated a significant interest in this field. Our aim is to evaluate a scalable intervention targeted at CCMBs, while also promoting psychological well-being associated with sustainable lifestyles.