Interpersonal exchanges may contribute to athletes’ motivational and well-being experiences through their contribution to athletes’ feelings of loneliness. Loneliness is understudied in sport, yet it is potentially salient in connecting social relationships with motivational processes and well-being of athletes. The purpose of the current research was to examine (a) the association of aspects of teammate relationships with athletes’ perceptions of burnout and engagement and (b) whether loneliness explained these associations. Adolescent athletes (N = 279) completed established measures of teammate relationships, loneliness, burnout, and engagement. The mediational model was invariant between boys and girls. Loneliness mediated the relationship of social support (β = −0.14, 0.10), corumination (β = 0.09, −0.06), and appraisal of peer rejection (β = 0.11, −0.08) with burnout and engagement, respectively. Continued examination of athletes’ loneliness will extend understanding of athletes’ motivational and well-being experiences and inform the promotion of adaptive sport experiences.