Parenting and children's temperament are important influences on language development. However, temperament may reflect prior parenting, and parenting effects may reflect genes common to parents and children. In 561 U.S. adoptees (57% male) and their birth and rearing parents (70% and 92% White, 13% and 4% African American, and 7% and 2% Latinx, respectively), this study demonstrated how genetic propensity for temperament affects language development, and how this relates to parenting. Genetic propensity for negative emotionality inversely predicted language at 27 months ([Beta] = -0.15) and evoked greater maternal warmth ([Beta] = 0.12), whereas propensity for surgency positively predicted language at 4.5 years ([Beta] = 0.20), especially when warmth was low. Parental warmth ([Beta] = 0.15) and sensitivity ([Beta] = 0.19) further contributed to language development, controlling for common gene effects.