Background Digital mental health interventions are increasingly used to scale up access to mental health support, yet very few mental health apps are empirically supported, with high attrition rates. The therapeutic alliance between therapists and clients is a key factor in predicting psychological therapy retention and outcomes. Understanding how this concept translates to the digital context, the so-called digital therapeutic alliance (DTA), may help enhance retention and outcomes in digital mental health. Objectives To explore and conceptualise the DTA with mental health interventions delivered by standalone apps. Specifically: (1) whether people with mental health difficulties form an alliance with mental health apps; (2) what constitutes the DTA; and (3) the similarities and differences across the traditional therapeutic alliance and DTA. Methods Qualitative semi-structured interviews were conducted with 17 individuals utilising mental health apps. Interview topic examples included rationale for app support, goals around app use and perceived connection. Interviews were analysed using thematic analysis. Results Five core themes were identified: (1) connection with ‘an other’; (2) accessibility; (3) choice and empowerment; (4) goals and expectations: having lower expectations than in-person support; and (5) safe place: non-judgemental, loyalty, trust. Conclusions The study strengthens support for the concept of the DTA but adds to existing knowledge by suggesting that the DTA needs to be conceived differently to the traditional therapeutic alliance. App developers can enhance alliance by paying particular attention to the use of human voice features, validating statements, personalised interfaces, and multiple options for activities and strategies.