Although both social support and cognitive appraisals are strong predictors of children’s posttraumatic adjustment, understanding of the interplay between these factors is limited. We assessed whether cognitive appraisals mediated the relationship between social support and symptom development, as predicted by cognitive models of posttraumatic stress disorder (PTSD). Ninety seven children (Mean age = 12.08 years) were assessed at one month and six months following a single incident trauma. We administered self-report measures of cognitive appraisals, social support, and a diagnostic interview for PTSD. Results indicated that cognitive appraisals at one month post-trauma mediated the relationship between social support at one month post-trauma, and PTSD severity at follow-up. Differences in this relationship were observed between child-reported social support and parent-rated ability to provide support. Firm evidence was provided for the application of cognitive models of PTSD to children.