<p>The increasingly hostile sociopolitical climate is heightening psychological distress among trans and nonbinary (TNB) youth, who already face elevated risks for depression, anxiety, suicidal ideation, and substance use. Parental support is a key protective factor for TNB youth to prevent these negative mental health outcomes. Yet, there are minimal existing evidence-based interventions that aim to increase parental support among parents and caregivers (PCs) of TNB youth. The Parent Support Program (PSP) is a theory-driven, asynchronous, online intervention that was developed and refined based on community and expert feedback. This study used a randomized controlled trial (RCT) with 619 PCs of TNB youth to test the effectiveness of the PSP. We applied linear regression models with multiple imputation adjustments and propensity weights to estimate the effects of being assigned to the intervention condition (intent to treat effects) and global dosage of the PSP (total number of activities completed). Results from our intent to treat (ITT) analyses found that the PSP was effective at increasing self-compassion and PCs’ self-rated level of knowledge about gender diversity. However, global dosage predicted self-rated knowledge and the use of affirming language, but not self-compassion. The PSP is a promising intervention that may be a useful resource for some PCs of TNB youth. We address limitations of the PSP and future directions of how the PSP could be further refined and tested to optimize the intervention’s effectiveness and implementation, as well as clinical implications.</p>