This study examines how men and women in heterosexual partnerships influence each other’s parental leave decisions through their gender role attitudes. We differentiate between attitudes toward women’s parental role, women’s worker role, men’s parental role, and men’s worker role, and consider the role of traditional gender ideology denoting an attitude of negatively evaluating mothers’ employment when children are young. We investigated communal traits as a potential moderator to better understand partner effects, i.e., one partner’s role attitudes affecting the other partner’s leave decision. We analyzed longitudinal data from N = 365 heterosexual, mainly German dual-earner couples, collected between pregnancy and about 18 months after the birth of their first child, using the actor-partner interdependence model. We examined mothers’ and fathers’ attitudes toward all five types of gender roles and found that both mothers and fathers were influenced in their leave decisions by their partners’ attitudes toward early maternal employment. Mothers whose partners were more traditional in this regard took longer leaves; fathers whose partners were more traditional took shorter leaves. Fathers’ leave length was also influenced by their partners’ attitudes toward men’s worker role, with more traditional attitudes resulting in shorter leaves. The latter relationship was moderated by fathers’ communal traits, such that more communal fathers were more strongly influenced by their female partners’ attitudes. Overall, this research extends the understanding of mutual influences and decision-making dynamics in dual-earner couples in the early family phase.