Background In cross‐sectional studies, social support and posttraumatic stress disorder (PTSD) symptoms appear related, in that higher severity of PTSD is associated with lower social support and vice versa. Theoretical models of the causal direction of this relationship differ. Most longitudinal studies suggest that PTSD symptoms erode social support over time, although some suggest that higher social support is prospectively associated with decrease in PTSD symptom severity. It is unclear, though, how social support and PTSD affect each other in the short term. The purpose of this study was to test day‐to‐day relationships between PTSD and social support to elucidate how PTSD and social support influence each other. Methods Using 1173 daily observations from 75 college women who met screening criteria for lifetime sexual assault and past‐month PTSD, this study tested same‐day and next‐day relationships between PTSD and social support using mixed models. Results Within‐person analyses indicated that, when PTSD was higher than usual on a given day, social support was higher the next day. Between‐person analyses suggested that people with generally higher social support tended to have lower PTSD symptoms on a given day, but average PTSD symptom severity was not associated with day‐to‐day fluctuations in social support. Conclusions Rather than eroding in response to daily symptoms, social support might be sought out following increases in PTSD, and when received consistently, might reduce symptoms of PTSD in the short term. Interventions that increase college women's access to social support after sexual assault may thus be helpful in addressing PTSD.