This study explored if adolescents' style of decision-making is related to the sex composition of their friendship groups. Using data on 13,413 members of the Millennium Cohort Study at ages 11 and 14 years, we explored reciprocal associations between decision-making, measured with the Cambridge Gambling Task, and own-sex and other- or mixed-sex companionship. Cross-lagged models showed that girls whose friends at 11 were mainly girls showed better quality of decision-making, more risk adjustment, shorter deliberation time and less delay aversion at age 14, compared to girls in mixed-sex or other-sex friendship groups at 11. For boys, having predominantly male friends was associated only with more risk adjustment. Conversely, decision-making style at age 11 did little to predict keeping own-sex company at age 14. It appears that same-sex friendships may help develop better decision-making in adolescence, but only for girls.