Citizen science is becoming increasingly popular as a format in environmental and sustainability education. Citizen science not only allows researchers to gather large amounts of biodiversity-related data, it also has the potential to engage the public in biodiversity research. Numerous citizen science projects have emerged that assume that participation in the project affects participants' knowledge, attitudes, and behavior. We investigated what evidence really exists about the outcomes of biodiversity citizen science projects on the side of the individual participants. For this purpose, we conducted a systematic review of peer-reviewed research articles published up to and including 2017. We found evidence for various individual participant outcomes. The outcome reported most often was a gain in knowledge. Other outcomes, found in several articles, referred to changes in behavior or attitudes. Outcomes reported less often were new skills, increased self-efficacy and interest, and a variety of other personal outcomes. We discuss the research design and methods used in the reviewed studies und formulate specific recommendations for future research. We conclude that citizen science is a promising option for environmental and sustainability education focusing on biodiversity. Partnerships between natural and social scientists in the design and evaluation of projects would allow future biodiversity citizen science projects to utilize their full educational potential.