Considerable research demonstrates a positive association between sexual satisfaction and relationship satisfaction, but longitudinal evidence on the direction of this link remains inconclusive. To address this research gap, the present research provided a stringent test of the within-person associations between sexual and relationship satisfaction over time by analyzing 4-year longitudinal data from a nationally representative sample of newlywed couples in the United States (N = 2,104). The results indicated that within-person changes in sexual satisfaction predicted future changes in relationship satisfaction, whereas changes in relationship satisfaction did not predict future changes in sexual satisfaction. These results remained consistent when accounting for changes in couples’ sexual frequency, which showed significant associations with sexual satisfaction but non-significant associations with relationship satisfaction over time. All associations were consistent across gender. Overall, the current findings inform theory and practice on the roles of sexual dynamics in shaping overall perceptions of intimate relationships.