The social-emotional functions associated with Machiavellianism have been widely examined. However, no study has explored the relations among Machiavellianism, internalizing problems, and externalizing problems in adolescents. In samples from Chinese adolescents (N = 413), we examined how Machiavellianism is related to internalizing and externalizing problems. Results indicated that (a) Machiavellianism was positively associated with hyperactivity, emotional symptoms, conduct problems, and peer relationship problems; (b) sex of the participant may moderate the relations between Machiavellianism and peer relationship problems, with the positive correlation is stronger for girls than for boys; (c) emotional symptoms partially mediated the effect of Machiavellianism on hyperactivity, and partially mediated the effect of Machiavellianism on conduct problems. Additionally, results also indicated girls scored significantly higher than boys on emotional symptoms, and there is a general tendency that boys reported higher Machiavellianism than girls.