We study how the diffusion of online dating platforms has shaped intermarriage patterns by race and education in the United States. Online dating can, in principle, foster heterogamous unions by broadening users’ exposure to diverse racial and educational groups, or alternatively reduce such unions by enabling more effective preference-based screening. Using data from the American Community Survey and a continuous-treatment difference-in-differences design, we find that online dating platforms significantly affect interracial marriage rates, though their impacts differ across platforms. In contrast, we find weaker evidence that online dating has altered educational homogamy. To explore the underlying mechanisms, we draw on an original survey where we collected respondents’ retrospective dating histories, partner preferences, and online dating behavior. Leveraging individual panel data regressions that account for time-invariant heterogeneity in selecting into online dating, we show that meeting a partner online is associated with a higher likelihood of interracial marriage—except among individuals with strong same-race preferences, who are more likely to use filters and marry within their group. Taken together, our findings reveal that the effects of online dating platforms on marriage formation hinge critically on both platform features and individual preferences.