The heightened levels of peer relationship difficulties associated with Attention Deficit/Hyperactivity Disorder (ADHD) potentially predispose adolescents to feelings of loneliness and depressive symptoms. The current study explores whether feelings of loneliness mediate the effects of ADHD on depressive symptoms. Eighty-four adolescents (Mage = 13.01 years, 75% Male) in Western Australian schools completed mental health and wellbeing surveys. Multivariate analysis of variance assessed whether adolescents with ADHD had greater loneliness and depressive symptoms, and mediation analysis explored whether loneliness mediated the relationship between ADHD and depressive symptoms. Adolescents with ADHD reported significantly greater depressive symptomatology and feelings of isolation and lower quality of friendships. Together, friendship and isolation related loneliness fully mediated the relationship between ADHD and depressive symptoms. Loneliness is associated with depression in adolescents with ADHD and may be an important consideration when addressing symptoms of depression among young people diagnosed with ADHD.