Social interaction difficulties in children with autism spectrum disorder can be challenging, especially during adolescence. In addition, high rates of comorbid internalizing disorders in autism spectrum disorder can lead to social isolation. With limited social activity participation, social deficits and internalizing problems in autism spectrum disorder may be related to participation, exposure, and practice with friends. The purpose of this study was to evaluate the relations among friendship, activity participation, and internalizing problems. Participants included 129 children with autism spectrum disorder between the ages of 6 and 18 years. Measures of friendship, sport, hobby, and club participation, and internalizing problems were assessed. Results showed that activity participation was related to more friendships, even after controlling for intelligence quotient. This study sheds light on the impact of social engagement with peers in developing and maintaining friendships as well as managing internalizing problems.