Estimates of the size of the multiracial population in the United States depend on what prompts people to report multiple races on censuses and surveys. We use data from the 2015 Pew Survey of Multiracial Adults to explore how racial self-identification is shaped by the generational locus of an individual’s multiracial ancestry—that is, the place in one’s family tree where the earliest interracial union appears. We develop the theoretical rationale for considering generational heterogeneity and provide its first empirical demonstration for U.S. adults, by estimating what shares of the population identify multiracial ancestry in their parents’ or grandparents’ generation, or further back in their family tree. We find that multiracial generation is related to—and likely confounded with—the ancestry combinations that individuals report (e.g., white-Asian, black–American Indian). Finally, we show that later generations are less likely than their first-generation counterparts to select multiple races when they self-identify. Consequently, we argue that generational locus of multiracial ancestry should be taken into account by demographers and researchers who study outcomes for multiracial Americans.