Online transdiagnostic (TD) self-help interventions address common processes underlying multiple psychological disorders, such as emotional dysregulation or behavioral avoidance. They may target specific clinical areas like anxiety and depression, as well as broader constructs like well-being. This chapter provides a conceptual explanation of the TD approach and its history, the current evidence base for its effectiveness, and a synopsis of therapies that have been adapted into online TD self-help interventions. Specific TD programs available in real-world contexts will be highlighted throughout the chapter as illustrative examples. We outline various therapeutic models adapted for online TD self-help, each of which demonstrates positive outcomes in both clinical and preventative contexts. Challenges such as engagement and adherence are addressed, along with strategies like personalization and tailoring for specific populations to improve relevance and accessibility. Future directions highlight the role of advancing technologies in enhancing TD self-help interventions. Overall, TD online self-help represents a promising, scalable solution for expanding access to effective mental health support.