Prejudice against or affirmation of gender/sex minorities is often framed in terms of beliefs about the ontology of gender/sex (i.e., what gender/sex is), or gender/sex diversity beliefs. We constructed the Gender/Sex Diversity Beliefs Scale (GSDB) to assess ontological beliefs about the nature of gender/sex, including essentialist and social constructionist beliefs, and validated the GSDB across a series of studies. In Study 1 ( N = 304), we explored the factor structure of the GSDB and found evidence of associations with prejudice against transgender and/or nonbinary people. In Study 2 ( N = 300), we assessed the stability of the factor structure of the GSDB and examined its criterion-related validity, including its relationship to feelings toward multiple gender/sex groups. In Studies 3a ( N = 48) and 3b ( N = 500), we established test–retest reliability. We conclude that gender/sex diversity beliefs are important for understanding contemporary attitudes about gender/sex, including prejudice against gender/sex minorities, and that the GSDB is a reliable and valid way to measure them.