This study examines externalizing symptoms (attention-deficit/hyperactivity disorder [ADHD], conduct problems, and callous-unemotional [CU] traits) in relation to domains of peer functioning (social competence, loneliness, and close friendship quality), with a specific focus on the role of CU traits. One hundred twenty-four elementary students (grades 3–6; 45% boys) completed multiple measures of peer functioning, and teachers completed measures of externalizing symptoms and social competence. After controlling for demographic variables and other externalizing symptoms, CU traits were significantly associated with poorer peer functioning across all variables except for demands of exclusivity in close friendships. ADHD symptoms were also uniquely associated with poorer social functioning across a number of variables. In contrast, conduct problems were at times associated with better social functioning after controlling for the effects of other externalizing problems. These findings bolster the importance of developing and evaluating social skills interventions for children displaying elevated CU traits.