The interpersonal-psychological theory of suicidal behavior (Joiner, 2005) proposes that the need to belong is fundamental; when met it can prevent suicide and when thwarted it can substantially increase the risk for suicide. We investigate one source of group-wide variation in belongingness among college students—changes in the social composition of college campuses across academic semesters—as an explanation for variation in suicidal ideation across the academic year. Our results indicate that in a sample of college students at a large southern state university (n = 309), suicidal ideation varied across academic semesters, with highest levels in summer compared to both spring and fall. Differences in suicidal ideation between summer and spring were, in large part, accounted for by belongingness. Theoretical, as well as practical, implications are discussed regarding mechanisms for seasonal variation in suicidal ideation.