Children with attention-deficit/hyperactivity disorder (ADHD) are well-documented to experience social-emotional difficulties; however, little is known about their loneliness—an aspect of social-emotional functioning. Using a cross-sectional design, we examined how loneliness relates to comorbid internalizing disorders, externalizing disorders, and peer problems in a sample of 213 children with ADHD. Children (66 girls, Mage = 8.58, SDage = 1.55) reported their loneliness. Comorbid internalizing and externalizing disorders were assessed via a multi-informant procedure. Proportion of classmates who accepted, rejected, and ignored the child, friendship quantity, and friendship quality were peer problem indicators. Results suggested that children with comorbid internalizing disorders, fewer friendships, or potentially more negative friendship quality, reported more loneliness. Gender appeared to moderate the association between peer rejection and loneliness, such that boys with peer rejection reported more loneliness than girls. Clinical implications include targeting loneliness as a social-emotional problem to assess and treat in children with ADHD.