This study focuses on the reliability of intra-group peer assessment (PA) of the collaboration processes in a wiki. First-year university students (N = 659) were subdivided in groups of 8 to create wikis. They were to assess their group members on four criteria (contribution, discussion, sources, and social). The main research question focused on the reliability of this intra-group peer assessment. Intraclass correlations were calculated and the results showed reliabilities with an intermediate to large effect size for the overall score. The main conclusion is that peer assessment is reliable and that it is feasible to implement in the context of online collaboration in a wiki environment. This leads to the practical implication that a web-based peer assessment tool can support instructors to manage computer-supported collaborative learning activities in a wiki-environment. (Contains 5 tables.)