Although the motivational capacity of serious games is well supported in theory, few experimental studies have been conducted so far. This study contributes to this knowledge gap by using a serious game to try and increase the currently low student retention in Massive Open Online Courses (MOOCs). For this, a serious game was specifically designed to influence factors that have shown to influence student retention in other MOOCs. A randomized post-test only control group experimental design was used, in which the impact of the serious game on student retention was evaluated with quantitative and qualitative data from questionnaires and the edX MOOC platform. While it was found that the serious game had a negative impact on the intrinsic motivation of students, it appeared that intrinsic motivation did not influence student retention in this MOOC, therewith explaining why no difference in student retention was observed between the control group and game group. This shows that in order to improve student retention in MOOCs with serious games, further research on factors affecting student retention in MOOCs is required first.