Youths' development can be positively or negatively impacted by various factors in their internal and external environment. This study investigated the interactive effects of self-regulation and internalizing and externalizing problems on academic achievement (reading and maths) and positive youth development (PYD) across one year. Data were obtained from 390 Icelandic youths (44.75% male) at two time points, in grades 6 (M[subscript age] = 11.71) and 7 (M[subscript age] = 12.70). Structural equation modelling indicated that self-regulation positively predicted all outcome measures one year later, whereas internalizing and externalizing symptoms negatively predicted PYD and maths. The findings further revealed that internalizing symptoms mediated the effects of self-regulation on maths performance, while externalizing symptoms mediated the effects of self-regulation on PYD. These results suggest that self-regulation not only predicts academic achievement and PYD directly, but may also influence these outcomes indirectly by decreasing internalizing and externalizing symptoms.