The current study evaluated the influence of a positive romantic partner on the intergenerational transmission of externalizing behavior across generations. The study included 213 generation one (G1) mothers and their adolescent (generation two; G2) who participated from middle adolescence through adulthood, G2’s romantic partner in adulthood, and the third-generation (G3) child between ages 3–5 years. Two steps were used to identify the role of G2’s romantic partner’s positive behavior on the transmission of externalizing behavior from G2 and G3. First, after controlling for G1 harsh parenting and G2 externalizing at Time 1, both G2 externalizing at Time 2 and G2 harsh parenting at Time 3 were associated with G3 externalizing at Time 3. Second, we found both main and interaction effects of G2 partner’s positive behavior with G2 externalizing behavior at Time 2 and G2 harsh parenting at Time 3 on G3 externalizing behavior at Time 3. Results suggest a positive romantic partner may have an important role in disrupting the negative effects of both G2 externalizing behavior and harsh parenting on their G3 child’s externalizing behavior.