Project-based learning (PjBL) engages students in knowledge acquisition, application, and construction through artefact development. Based on the Community of Inquiry framework, this study characterized college students' social and cognitive presences in online PjBL and examined how presence was related to their academic performance. Twenty-four groups of students participated in a 3-week project via WeChat discussion groups and created a final product. Transcripts of students' online discourse were collected and analysed by a coding scheme. The quality of students' artefacts was evaluated by a grading rubric. Descriptive results showed that the component of affectiveness and the level of exploration accounted for the majority of students' social and cognitive presences, respectively. Stepwise regression analyses revealed that certain components and sub-components of students' social presence, and levels and sub-levels of their cognitive presence were positively associated with their academic performance. Practical implications for teachers and suggestions for further research are provided.