1. Citizen science (CS) is regarded a promising format in environmental and sustainability education as well as in science education. CS projects often assume that participation in the project influences, for example, participants' knowledge or behaviour.

2. We investigated whether and to what extent biodiversity citizen science (BDCS) projects, from the participants' self-reported perspective, achieve the following six participant outcomes: (1) content, process, and nature of science knowledge, (2) skills of science inquiry, (3) self-efficacy for science and the environment, (4) interest in science and the environment, (5) motivation for science and the environment, and (6) behaviour towards the environment.

3. For this purpose, we conducted an online survey of 1,160 CS participants across 63 BDCS projects in Europe, Australia, and New Zealand. Our survey was aimed at adults participating in CS voluntarily.

4. Survey respondents reported positive changes in all six categories. The most notable result across projects was that self-reported increases in knowledge, self-efficacy, interest, and motivation were found to be more pronounced when regarding the environment rather than science. Perceived gains in data collection skills were reported to be higher than gains in skills not directly connected to data collection. Reported behaviour changes primarily concerned communication activities, to a lesser degree also gardening activities, and finally more general environmental behaviour.

5. In addition to these six participant outcomes, respondents mentioned a variety of other positive and negative outcomes, for example, health and wellbeing, enjoyment, a sense of satisfaction, an increased connection to people and nature, but also a more pessimistic view regarding the future of the environment.

6. We conclude that BDCS projects could have a high potential for environmental and sustainability education as well as science education. Further research should investigate individual participant outcomes in more depth and should focus on the factors that influence these participant outcomes. Moreover, exploring the perspectives of both project participants and project coordinators would be valuable. In this way, it would be possible to improve the development and design of CS projects. As a result, BDCS projects could more effectively achieve outcomes for the participants, for science, and for biodiversity.

This dataset can be opened with SPSS software.