Transgender and gender-expansive (TGE) youth endure stark disparities in health and wellbeing compared to their cisgender peers. A key social determinant of health for TGE adolescents and emerging adults is gender affirmation, which encompasses multidimensional validations of an individual's lived gender. Lacking available resources for one's gender affirmation, TGE young people may engage in highrisk maladaptive coping behaviors, linked to their disproportionately high HIV-acquisition risk. A range of innovative mobile technologies are guided by the Gender-Affirmative Framework to promote the health of TGE communities, including through HIV prevention and care continuum outcomes. The aim of this review was to examine key features of existing mobile technologies that can be leveraged to advance the field of TGE-responsive mHealth. We systematically searched scientific records, gray literature, and the iOS and Android app distribution services. To be eligible, platforms and interventions needed to be tailored exclusively to a TGE user base, incorporate gender-affirming features, and be optimized for or adaptive to mobile technologies. Eligible interventions (N=24) were compared on evidence of utility, core functionalities, and dimensions of gender affirmation. Smartphone applications (apps) and webapps (n=16) were the most common delivery modality. Many interventions (n=9) aimed to address HIV-related outcomes and integrated gender-affirmative features. The most common gender-affirmative features originated in fields of human-computer interactions and informatics, or were crowdfunded by TGE developers. HIV-focused interventions incorporated evidence-based health behavior change strategies and utilized rigorous evaluation methods. Across modalities and disciplines, behavioral self-monitoring and access to HIV prevention services were the most frequent features. Over two-thirds of the interventions reviewed aimed to provide medical gender affirmation (e.g, provided guidance on obtaining medically sanctioned hormone therapies, or safely practicing non-medical options such as chest-binding) or psychological gender affirmation (e.g, provided linkage to mental health counseling). Our results show that mHealth and other technology-mediated interventions offer a diverse range of both evidence-based and innovative features; however, many have not been rigorously evaluated in a randomized controlled trial to support TGE users. A continuing commitment to evidence-based health behavior change strategies, exemplified by the HIV-focused interventions included in this review, is essential to advancing gender-affirmative mHealth. The unique and highly innovative features of platforms originating outside the fields of HIV prevention and care suggest new directions for TGE-responsive mHealth, and the need for more conscientious models of knowledge exchange with investigators across scientific disciplines, private-sector developers, and potential users. mHealth, 2021