Loneliness has been associated with several adverse psychosocial outcomes in childhood and adolescence. The present study aimed to investigate feelings of loneliness and social dissatisfaction in school-aged typically-developing (TD) siblings of children with an autism spectrum disorder (ASD). For this purpose, 118 siblings of children with ASD and 115 siblings of TD children and one of their parents participated in this study. Siblings of ASD-children reported higher levels of loneliness and social dissatisfaction than the controls. The hierarchical multiple regressions performed revealed that those feelings were inversely associated with being the first-born and with specific aspects of social support as perceived by the parent. The younger siblings of ASD-children seem to be in need of certain interventions beyond social support.