Sense of belonging is associated with postsecondary student success outcomes and has largely been studied within face-to-face course contexts. The increasing demand for online courses after the COVID-19 pandemic necessitates identifying ways instructors can foster belonging in their online courses. This study experimentally tested the effect of using social annotation on students' sense of belonging in asynchronous online courses. Students in two psychology courses (N = 44) were randomly assigned to complete annotation assignments individually (control) or in groups using a social annotation platform (treatment). Students completed a course-level sense of belonging measure at the end of their courses and indicated their perceptions of social annotation. Sense of belonging was significantly higher in the social annotation condition, particularly for the introductory-level psychology course, and students reported positive perceptions of social annotation. Course performance did not differ between annotation conditions, and sense of belonging was not associated with final grades. These findings document the viability of social annotation as an evidence-based tool for promoting a sense of belonging in online psychology courses.