<p>System-justification is the tendency to justify and preserve different sociopolitical systems, which can affect even the ones harmed by them. Based on previously developed scales such as the General System-Justification Scale and the Gender-Specific System-Justification Scale, this article proposes and validate through five different studies a new Heteronormative System-Justification Scale that measures the degree to which individuals justify and legitimate the heteronormative system, that is, the position of inferiority and subordination LGBs currently occupy in society. The studies were conducted in the Brazilian context with sexual minorities, a country in which these individuals are still widely discriminated, alike other Latin-American countries. In Study 1, we calculate a content validity coefficient based on experts’ analyses. In Study 2 (<i>n</i> = 376), we conducted exploratory factor analysis and found a single-factor structure. In Study 3 (<i>n</i> = 397), this structure is confirmed through a different sample. In Study 4 (<i>n</i> = 777), we present convergent and incremental validity of the heteronormative system-justification by comparing it with empirically related constructs. At last, in Study 5 (<i>n</i> = 662), we conducted a social experiment, exposing participants to system-justifying and system-critical information to manipulate heteronormative system-justification and test its causal effect on subjective well-being. Overall, these studies present several different evidences of validity and reliability for the scale, which can be used to further investigate system-justification tendencies among sexual minorities and correlated sociopolitical and psychological processes.</p>