Women perform less physical activity (PA) than men, and this gap widens during college. This study examined college women’s daily PA intentions and behavior, and whether social support or social comparison orientation (SCO) moderated the PA intention-behavior relation. College women (N = 80) completed measures of social support and SCO at baseline. For seven consecutive days, participants completed an electronic survey to assess PA intentions and wore an activity monitor to assess minutes of moderate-to-vigorous intensity physical activity (MVPA). Results indicated that intended and performed MVPA minutes were weakly related (p = 0.17, sr = 0.16). Social support did not moderate the intention-behavior relation, but SCO did (p = 0.04, sr = 0.21). Participants with stronger (vs. weaker) SCO, particularly a tendency to compare downward (i.e., to worse-off others), showed smaller discrepancies between intended and completed MVPA. College women frequently fail to achieve PA goals, but stronger tendencies to make (downward) social comparisons may minimize this gap and be a target for intervention.