Previous research suggests that acute pain is a risk factor for later posttraumatic stress symptoms (PTSS). In a prospective cohort study, we examined the association between acute pain from accidental injury and PTSS in children and adolescents, taking into account factors potentially related to pain or posttraumatic stress. Participants were 135 children and adolescents, 8–18 years old. We measured the worst experienced pain since the accident took place with a visual analogue scale. Three months after the accident, posttraumatic stress was assessed with a self-report measure. We found a positive association between acute pain and posttraumatic stress. The amount of pain was negatively associated with injury severity in girls and positively associated with the presence of an extremity fracture in boys. In children who reported severe pain, this pain was significantly associated with PTSS and may account for around 10% of the variance in the severity of PTSS. Although the experience of pain is subjective, our study indicates that severe pain is associated with the severity of later PTSS. Timely management of pain according to acute pain protocols in all phases and disciplines after accidental injury is therefore recommended.