It is important to understand factors associated with sport participation for youth with intellectual and developmental disabilities (IDD). With a sample of 414 Special Olympics (SO) athletes, this study examined how frequently involved athletes differ from other youth who are less involved in SO. Results showed that frequently involved athletes are older, have more sport-specific parental support, stronger athlete-coach relationships, and more positive SO experiences than other athletes. These factors were predictive of SO involvement, even after controlling for athlete characteristics, including behavior problems and adaptive behavior. Athletes with IDD have the potential to be highly involved in sports when external supports (i.e., coaches and parents) are strong.