Abstract: Posttraumatic growth (PTG) is characterized as the experience of positive psychological change following exposure to traumatic stress. However, studies examining the association between posttraumatic stress disorder (PTSD) symptoms and PTG have demonstrated mixed results. Further, although higher levels of social support have been shown to predict higher ratings of PTG, there are limited longitudinal findings regarding how interpersonal functioning may affect the association between PTSD symptoms and PTG. In this study, we examined interpersonal functioning in romantic relationships and friendships as mediators of the relation between PTSD symptom severity and PTG and examined potential sex differences among these associations. Participants were 1,427 veterans who completed self‐report questionnaires across three time points. A parallel mediation analysis indicated that romantic relationship, β = −.01, 95% CI [−.03, −.001], and friendship, β = −.02, 95% CI [−.04, −.001], functioning were both indirectly associated with PTG in the full sample. Moreover, sex‐stratified models indicated that romantic relationship functioning was a significant mediator for male veterans, β = −.02, 95% CI [−.05, −.01], whereas friendship functioning was a significant mediator for female veterans, β = −.04, 95% CI [−.08, −.01]. Together, these findings suggest that interpersonal functioning is an important factor in PTG, and targeting romantic relationship and friendship functioning in the context of trauma‐focused treatment may be valuable and can help build a pathway from PTSD symptoms to PTG among female and male veterans.