<p>Although offspring of mothers with major depressive disorder (MDD) are at heightened risk for the disorder, most never develop depression, highlighting the need to identify moderators of risk. Co-rumination, the tendency to frequently and passively discuss problems with a peer without any active problem-solving, is a known risk factor in youth and may help to identify which offspring of mothers with MDD are at greatest risk. We tested this hypothesis using data from a multi-wave longitudinal study, examining prospective changes in youths’ depressive symptoms and risk for MDD onset during the two-year follow-up. Participants were 214 children (ages 8–14 at baseline) of mothers with (<i>n</i> = 106) or without (<i>n</i> = 108) a history of MDD. At baseline, mothers completed a diagnostic interview, and children completed a self-report measure of co-rumination. Follow up assessments, completed every 6 months for 2 years, were used to assess depressive symptoms and the onset of MDD episodes in children. Supporting our hypothesis, among offspring of mothers with MDD, but not among offspring of never depressed mothers, higher levels of co-rumination at the baseline assessment predicted higher levels of depressive symptoms in offspring at baseline the assessment that were maintained through the end of the follow-up period, as well as increased risk for the onset of MDD across the follow-up. These results highlight the role of co-rumination, which is a modifiable risk factor that may be targeted in prevention and early intervention programs to reduce risk for the intergenerational transmission of depression.</p>