Irritability is a common feature of many psychiatric disorders, including both externalizing and internalizing disorders. There is little research, however, examining associations between irritability and these symptom domains, particularly during the important developmental period of adolescence, characterized by sex differences in the prevalence of disorders. We examined the cross-sectional associations between irritability, measured with the Affective Reactivity Index, and symptoms of externalizing and internalizing domains of psychopathology, measured with the Youth Self Report, in a volunteer community sample (N = 183) of 9- to 13-year-old (M = 11.39, SD = 1.07) boys and girls (37% White/Caucasian, 8% Asian, 11% Hispanic, 8% African American, 2% Native American, 2% Pacific Islander, 28% Other, and 3% not reported). A subset of the sample (n = 112) provided data at a 2-year follow-up, used to extend these associations. There were no sex differences in levels of irritability; however, the associations between irritability and symptom domains were moderated by sex. Specifically, in girls, irritability was associated equally with externalizing and internalizing symptoms. In contrast, in boys, irritability was associated more strongly with externalizing symptoms than with internalizing symptoms. Thus, across both sexes, irritability was moderately associated with externalizing symptoms, but the association between irritability and internalizing symptoms was stronger in girls than in boys. At follow-up, sex moderated the association between baseline irritability and later externalizing and internalizing symptoms. These findings indicate that irritability is associated with both externalizing and internalizing symptoms in early adolescence and that irritability is associated with internalizing symptoms more strongly in girls than in boys.