Parental support is instrumental in protecting the mental health of trans and nonbinary youth (TNB). Yet minimal measures exist that measure parental support, and current measures typically measure parental attitudes or general perceptions of supportiveness rather than measuring specific TNB-affirming behaviors. This study aimed to fill this gap by developing and validating the Trans- and Nonbinary-Affirming Parental Practices scale. The scale items were developed based on existing psychological literature on TNB-affirming parenting behaviors as well as adapting existing measures. The sample consisted of 570 parents or caregivers of TNB youth ages 5-27 (M = 15). The vast majority of participants were White (90%) and cisgender women (92%). The sample was randomly split to conduct both exploratory factor analysis and confirmatory factor analysis. Factor analysis found good fit for a four-factor structure with 14 items. Each subscale represented a type of TNB-affirming parental behavior: communicating support, affirming language, advocating for their TNB child, and seeking affirming resources. The measure demonstrated invariance with both heterosexual participants and participants with minoritized sexual identities and demonstrated convergent and discriminant validity. The Trans- and Nonbinary-Affirming Parental Practices measure can be useful in intervention research aimed at increasing TNB-affirming behaviors among parents and caregivers of TNB youth. (PsycInfo Database Record (c) 2024 APA, all rights reserved).