Background <p>Therapeutic alliance is an important aspect of psychotherapy and has been associated with treatment outcomes, but less is known about how it is experienced and maintained in guided internet-delivered cognitive behavioural therapy (iCBT), particularly when treatment concerns persistent physical symptoms such as non-cardiac chest pain (NCCP). This study explored how participants in an 8-week guided iCBT programme for NCCP experienced the therapeutic alliance, and how their accounts related to established alliance models.</p> Methods <p>This qualitative interview study was conducted alongside a randomised controlled trial evaluating an eight-week guided iCBT programme for adults with NCCP and cardiac anxiety. Semi-structured interviews were conducted with 22 participants who had completed the programme. The interviews were audio-recorded, transcribed verbatim, and analysed using reflexive thematic analysis informed by Bordin’s model of therapeutic alliance, while remaining open to inductive theme development. Descriptive statistics were used to summarise participant characteristics.</p> Results <p>The analysis generated three themes. First, agreement on goals and tasks was shaped by whether participants experienced the programme as personally relevant and whether treatment tasks could be balanced between fit and challenge. Second, the emotional bond between participant and therapist was supported by timely, attuned, and appropriately calibrated written feedback, although participants differed in their preferred level of emotional closeness and professional distance. Third, acceptability of the digital medium for collaboration encompassed the delivery format, the online platform, and the treatment content. Across themes, participants described alliance in the digital context as shaped by processes of negotiation in which therapist responsiveness helped them make sense of the treatment rationale, manage strain, and sustain engagement.</p> Conclusions <p>In guided internet-delivered cognitive behavioural therapy for non-cardiac chest pain, therapeutic alliance appeared to depend not only on agreement about goals, tasks, and emotional bond, but also on the acceptability of the digital medium through which collaboration occurred. The findings suggest that therapists delivering text-based internet interventions may need to actively calibrate task demands, relational tone, and symptom explanations to support engagement and collaboration.</p> Trial registration <p>This qualitative study was conducted within the registered parent trial: ClinicalTrials.gov NCT06136494; registered on November 13, 2023.</p>