Drawing on the first national survey experiment of its kind (n = 3,922), the authors examine Americans’ perceptions of transgender people’s sex and the factors that underlie these perceptions. The authors randomly assigned respondents to a vignette condition describing a transgender person whose self-identified gender (i.e., identifies as a man or a woman), age (i.e., adult or teenager), and gender conformity in physical appearance (i.e., conforming, nonconforming, ambiguous, or unspecified) had been experimentally manipulated. Then, respondents were asked how they would personally classify that person’s sex. The findings suggest that Americans are more likely to perceive a transgender person’s sex as consistent with their sex assigned at birth than with their gender identity. Furthermore, of the experimental manipulations included in the experiment, only the transgender person’s level of gender conformity—not their self-identified gender or age—affects public perceptions of sex. The authors also find distinct cleavages along sociodemographic lines, including politics, sexual orientation, and interpersonal contact with transgender people. Implications for research on sex and gender are discussed.