The current, dramatic biodiversity decline is a serious problem. In order to deal efficiently with it, stakeholders and society need to acknowledge and be aware of this problem. This could be fostered by engaging the public at large in biodiversity research activities. One way to do so is to involve citizens in citizen science (CS) projects. These are projects in which researchers collaborate with volunteering citizens in scientific research projects. Yet, it remains unclear whether engaging in such projects has an impact on the citizens who participate. Previous research has so far presented mixed results about the improvement of citizens’ attitudes and knowledge, mostly because this research has focused only on certain aspects of CS projects. To address these limitations, we investigated the effectiveness of a CS project on urban bat ecology regarding citizens’ attitudes toward bats, knowledge about bats, and attitudes toward engagement in CS. We also examined whether the degree of citizens’ participation had an influence on the outcomes. We conducted four field studies in this CS project on urban bat ecology using an experimental pre-post-measurement design. To manipulate the degree of participation, we assessed the post-measurement in one group directly after data collection, while in a second group, we assessed it after data collection and data analysis at the end of the project. Across all studies, the results demonstrated that citizens’ attitudes toward bats improved over time, their content knowledge of urban bat ecology increased over time, and their attitudes toward engagement in CS improved over time. Citizens’ degree of participation did not influence these outcomes. Thus, our research illustrates the effectiveness of CS for increasing awareness for urban bat conservation independently of citizens’ degree of participation. We discuss the implications of our findings for the CS community.