Background: Children with attention-deficit hyperactivity disorder (ADHD) experience substantial difficulty maintaining meaningful friendships, which has implications for social functioning and mental health. No systematic review has investigated their friendship difficulties. Objectives: To systematically review and methodologically appraise the quality of existing studies reporting on friendships of children with ADHD. To compare their friendships to typically-developing children, and examine associations between friendship and children’s social-emotional wellbeing and mental health. Method: Six databases were searched. The methodological quality of studies was assessed using the QualSyst appraisal tool and the Appraisal tool for Cross-Sectional Studies. Aspects of friendships measured were charted, along with comparisons between children with ADHD and typically-developing children and the associations between friendships and social-emotional wellbeing and mental health. Results: Twenty-three cross-sectional studies and one longitudinal follow-up study were included. Studies included 1509 participants with ADHD, with 1197 typically-developing participants used as a companion in 19 of the 24 studies. Friendship quantity was the most investigated aspect of friendship. Children and youth with ADHD had significantly fewer friends, lower quality friendships and poorer friendship interactions. There were mixed findings from studies investigating the role or impact of friendship on social-emotional wellbeing and mental health. Twenty-two had strong methodological quality. Conclusion: Limited longitudinal studies, small sample sizes and variability in measurement restrict the interpretations of friendship over time and the causal impact of friendship on social and emotional outcomes. Further research should investigate the role and impact of friendships on the social-emotional wellbeing of children and youth with ADHD.