Objectives Data sharing is well established in biological research, but evidence on sharing of clinical trial or public health research study data remains limited, in particular studies of research participants' perspectives of data sharing. This study systematically reviewed international evidence of research participants' attitudes towards the sharing of data for secondary research use. Methods Systematic search of seven databases, and author-, citation- and bibliography-follow up to identify studies examining research participants' attitudes towards data sharing. Studies were thematically analysed using NVivo v10 to identify recurring themes. Results Nine studies were eligible for inclusion. Thematic analysis identified four key themes: (1) benefits of data sharing, including benefit to participants or immediate community, benefits to the public and benefits to science or research; (2) fears and harms, such as fear of exploitation, stigmatization or repercussions, alongside concerns about confidentiality and misuse of data; (3) data sharing processes, in particular the role of consent in the process; and (4) the relationship between participants and research such as trust in different types of research or organization and the relationship with the original research team. Conclusions The available literature on attitudes towards sharing data from clinical trials or public health interventions remains scant. This study has identified four themes regarding research participants' attitudes and preferences, which should be considered by policy makers, and explored with further research.