Background: Informal caregivers represent an important and vulnerable part of the society. They can experience negative psychological symptoms, such as depression and anxiety. Internet-based cognitive behavioural therapy (ICBT) is a promising psychological support option that could be effective in reducing informal caregiver burden as well as improving psychological well-being. Qualitative studies are valuable for gaining insights into participant experiences of using ICBT. Objective: The main aim of this study was to explore informal caregiver experiences and hence acceptability and feasibility of the transdiagnostic ICBT program aimed at reducing informal caregiver burden and increasing the quality of life. Methods: Following a strategic sampling procedure, 23 informal caregivers were recruited from previous randomized controlled trial for informal caregivers in Lithuania. Participants were interviewed over phone, using semi-structured interview questions. Interviews were analysed using a thematic analysis approach. Results: Four themes and a total of 10 sub-themes were generated: I A program as a means of change (Convenience and applicability of the format and materials; Ability to focus on own needs; Opportunity for communication), II Suggestions for the program (Including live support; Tailoring materials and format; Providing with more time and resources), III Driving personal and situational forces (Developing acceptance and adjustment over time; Being proactive and/or receiving support) and Hindering situational factors (Deterioration and unpredictability of the care-receivers health; Lacking external support and opportunities for respite). Conclusions: Most of the informal caregivers were found to be satisfied with the program's format and materials as well as the communication with the therapist via a message function in the program. Some suggestions were made regarding implementation of the live support option. Also, suggestions regarding possibility for tailoring the program's content. Lastly, several personal and situational factors were identified as important in affecting informal caregiver well-being. We conclude that ICBT has potential in reducing informal caregiver burden and improving psychological health. Further research trials are warranted for evaluating both, the effectiveness and the feasibility of the program.