This study introduces a new instrument designed to assess affirmative clinical practices with sexual and gender minority (SGM) clients, the Lesbian, Gay, Bisexual, and Transgender Competency Assessment Tool (LGBT-CAT). The LGBT-CAT has two unique qualities: Its design enables adaptation to measure practice competencies with gay, lesbian, bisexual and/or transgender clients, the latter of which has no existing measures regarding practice competencies, and it uses a qualitative collection method. Participants respond to 12 open-ended prompts; responses are then quantified by one or more raters based on a scoring rubric. In this cross-sectional study, practicing social workers ( = 357) were surveyed using the LGBT-CAT as well as measures of affirmative practices with SGM clients, knowledge, beliefs, self-efficacy skills, attitudes, and behaviors. The LGBT-CAT demonstrated good reliability, poor criterion validity, and adequate construct validity. These results support the potential integration of the LGBT-CAT into research on practice with SGM clients.