Sexual and gender minority youth are at greater risk of compromised mental health than their heterosexual and cisgender peers. This is considered to be due to an increased burden of stigma, discrimination, or bullying resulting in a heightened experience of daily stress. Given the increasing digital accessibility and a strong preference for web-based support among sexual and gender minority youth, digital interventions are a key means to provide support to maintain their well-being. This paper aims to explicate the co-design processes and underpinning logic of Oneself, a bespoke web-based intervention for sexual and gender minority youth. This study followed a 6-stage process set out by Hagen et al (identify, define, position, concept, create, and use), incorporating a systematic scoping review of existing evidence, focus groups with 4 stakeholder groups (ie, sexual and gender minority youth, professionals who directly support them, parents, and UK public health service commissioners), a series of co-design workshops and web-based consultations with sexual and gender minority youth, the appointment of a digital development company, and young adult sexual and gender minority contributors to create content grounded in authentic experiences. Oneself features a welcome and home page, including a free accessible to all animation explaining the importance of using appropriate pronouns and the opportunity to create a user account and log-in to access further free content. Creating an account provides an opportunity (for the user and the research team) to record engagement, assess users' well-being, and track progress through the available content. There are three sections of content in Oneself focused on the priority topics identified through co-design: (1) coming out and doing so safely; (2) managing school, including homophobic, biphobic, or transphobic bullying or similar; and (3) dealing with parents and families, especially unsupportive family members, including parents or caregivers. Oneself's content focuses on identifying these as topic areas and providing potential resources to assist sexual and gender minority youth in coping with these areas. For instance, Oneself drew on therapeutic concepts such as cognitive reframing, stress reduction, and problem-solving techniques. There is also a section containing relaxation exercises, a section with links to other recommended support and resources, and a downloads section with more detailed techniques and strategies for improving well-being. This study contributes to research by opening up the black box of intervention development. It shows how Oneself is underpinned by a logic that can support future development and evaluation and includes diverse co-designers. More interactive techniques to support well-being would be beneficial for further development. Additional content specific to a wider range of intersecting identities (such as care-experienced Asian sexual and gender minority youth from a minority faith background) would also be beneficial in future Oneself developments. RR2-10.2196/31036.