Studies have shown that early language difficulties are associated with later internalizing problems. Less is known about the nature of the association: the bidirectional relationship over time, the role of different types of language difficulties, and gender differences. The present study examined bidirectional longitudinal associations between parent-rated language difficulties and internalizing problems in a four-wave cross-lagged model from 18 months to 8 years. Data from the Norwegian Mother and Child Cohort Study were used (N = 114,000). Gender-specific dichotomized language variables were created, and associations were investigated uniquely for boys and girls. Logistic regression analyses showed that all cross-lagged associations from 18 months to 5 years were significant for girls (odds ratios [ORs] = 1.48–1.94). For boys, only internalizing problems at 3 years predicted change in language difficulties (OR = 2.33). From 5 to 8 years, the cross-lagged associations between semantic language difficulties and internalizing problems were significant and strong for girls (ORs = 1.92–2.97) and nonsignificant for boys. The results suggest that the associations between language difficulties and internalizing problems are bidirectional from an early age, and that girls are especially vulnerable for developing co-occurring language difficulties and internalizing problems during the years of transition to school.