Autism spectrum disorder (ASD) as well as social phobia (SP), and selective mutism (SM) are characterised by impaired social interaction. We assessed the validity of the Social Responsiveness Scale (SRS) to differentiate between ASD, and SP/SM. Raw scores were compared in 6-18 year old individuals with ASD (N = 60), SP (N = 38), SM (N = 43), and typically developed (N = 42). Sensitivity and specificity were examined. The three disorders showed overlapping SRS scores. Especially in boys with SM (ROC-AUC = 0.81), presence of ASD was overestimated by the SRS. A combination of three disorder specific questionnaires resulted in marginally improved diagnostic accuracy. For the clinically very relevant differential diagnosis of SP/SM, SRS results must be interpreted with caution.