This study aims to develop and validate the Gender Stereotypes in Sport Scale (GSSS), an instrument evaluating people's endorsement of gender stereotypes in sport with a focus on soccer. The study was conducted with a total sample of 740 Italians, including 389 women and 351 men aged 18 to over 60, 403 laypeople and 337 experts in the field (i.e., athletes, coaches, medical staff, and managers). First, a pool of 22 items was initially created after conducting interviews with the experts. Next, the factorial structure of the scale was assessed through cross-validated exploratory-confirmatory factor analyses with two different sub-samples of 370 participants each. Three factors for a total of 16 items emerged from the analyses: "stereotypical beliefs" (i.e., stereotypical opinions about women and girls in soccer), "egalitarian beliefs" (i.e., items emphasising the equality of male and female performances), and "openness" (i.e., items concerning people's open-mindedness about women and girls in soccer). The validity of the GSSS was supported by significant correlations with sexism and adherence to masculine traits. Men, those with lower education, and experts reported higher endorsement of gender stereotypes according to all three factors. Crucially, multigroup confirmatory factor analysis confirmed scale invariance across gender, education, and familiarity with soccer. The GSSS allows researchers to assess gender stereotypes in sport, facilitating the understanding of their correlates. By better comprehending the causes and consequences of these beliefs, scholars and practitioners can contribute to ongoing public discussions regarding gender equity and promote inclusivity in the sport domain.