AbstractAlthough are several reliable measures used to assess outness levels, they were developed and validated for lesbian, gay, and bisexual adults. Using a national dataset of sexual and gender minority youth (SGMY) ages 13–17 (M = 15.57, SD = 1.27), this study examined the psychometric properties of a new outness measure to assess its structure and validity. This data source is unique in that it allowed for strategic comparisons among three subpopulations: cisgender sexual minority youth (CSMY), youth with both sexual and gender minority identifications (SMGMY), and heterosexual gender minority youth (HGMY). Accordingly, this study examined the measurement invariance of a sexual minority (SM) outness measure to assess if the same latent constructs are measured when it is completed by CSMY and SMGMY (i.e., sexual identity outness), as well as a gender minority (GM) outness measure when completed by HGMY and SMGMY (i.e., gender identity outness). Factor analysis results reveal that the measure accurately assesses gender and sexual identity outness. Measurement invariance results confirm that comparisons can be made between CSMY and SMGMY as well as between HGMY and SMGMY. This analysis highlights an opportunity to improve how outness is assessed and how it may function differently between SM and GM communities and for youth compared to adults.