BACKGROUND: Action mechanisms of therapeutic alliance in stepped and digital interventions remain unclear. AIMS: (a) To compare the development of therapeutic alliance between psychosocial treatment as usual (PTAU) and a stepped digital intervention designed to prevent distress in cancer patients; (b) to analyse the level of agreement between patients' and therapists' therapeutic alliance ratings; and (c) to explore variables associated with therapeutic alliance in the digital intervention. METHOD: A multicentre randomised controlled trial with 184 newly diagnosed breast cancer women was conducted. Patients were assigned to digital intervention or PTAU. Therapeutic alliance was assessed at 3, 6 and 12 months after inclusion using the working alliance inventory for patients and therapists. Age, usability (system usability scale), satisfaction (visual analogue scale), type and amount of patient-therapist communication were analysed as associated variables. RESULTS: Patients and therapists established high therapeutic alliance in the digital intervention, although significantly lower compared with PTAU. The development of patients' therapeutic alliance did not differ between interventions, unlike that of the therapists. No agreement was found between patients' and therapists' therapeutic alliance ratings. Patients' therapeutic alliance was associated with usability and satisfaction with app, whereas therapists' therapeutic alliance was associated with satisfaction with monitoring platform. CONCLUSIONS: A stepped digital intervention for cancer patients could develop and maintain strong therapeutic alliance. Neither the type nor amount of communication affected patients' therapeutic alliance, suggesting that flexible and available digital communication fosters a sense of care and connection. The association between usability and satisfaction with digital tools highlights their importance as key therapeutic alliance components in digital settings.