In citizen science (CS) projects, citizens who are not professional scientists participate in scientific research. Besides serving research purposes, CS projects provide participants opportunities for inquiry-based learning to promote their topic-specific knowledge and scientific reasoning skills. Previous research suggests that participants need scientific reasoning skills to engage in scientific activities and to learn from inquiry in CS projects. Participants' scientific reasoning skills, therefore, might enhance the resulting topic-specific knowledge at the end of a CS project. On the other hand, scientific reasoning skills themselves are a learning outcome of CS projects. Hence, they might play a double role in CS projects: as a learning outcome and as a prerequisite for acquiring knowledge. In the informal education context of CS, it has not yet been investigated whether scientific reasoning skills predict topic-specific knowledge or vice versa. To address this question, the research presented here used a cross-lagged panel design in two longitudinal field studies of a CS project on urban wildlife ecology (N = 144 participants). The results indicated that participants' scientific reasoning skills positively influenced their topic-specific knowledge at the end of the project, but not vice versa. Extending previous research on individual learning outcomes of CS projects, the results showed that inquiry-based learning in CS projects depends on certain prerequisites, such as participants' proficiency in scientific reasoning. We discuss the implications for future research on inquiry-based learning in CS projects and for further training of CS participants in acquiring scientific reasoning skills.