Objective: Although Animal-Assisted Interventions (AAI) are effective treatments for a variety of psychological problems, the mechanism of treatment effectiveness remains unclear. Qualitative studies of AAI may reveal possible mechanisms. This review aims to synthesize qualitative research and identify factors that might contribute to the effectiveness of AAI. Methods: A literature search of qualitative evidence published before August 8, 2018 was conducted using the following databases: PubMed, ERIC, PsycARTICLES, PsycINFO, and HABRI, with the aim of identifying qualitative research conducted with individuals undergoing AAI. Quality assessment was undertaken by CASP and the certainty of the evidence was evaluated using CERQual. Results: A total of 1866 articles were reviewed, and seven were included in the final analysis. A total of six themes were identified as factors relating to the effectiveness of AAI: 1. Fostering feelings of normalcy, 2. Improving behavioral activation, 3. Self-esteem enhancement, 4. Physical contact, belonging, and companionship, 5. Calming and comforting, and 6. Distraction. Barriers to AAI effectiveness were also identified. Conclusion: The results of these studies suggest that AAI was viewed as a positive and highly accepted intervention across populations and settings. AAI might be a useful intervention among people who suffer from a variety of mental disorders. All themes consistently demonstrated that contact with a live animal is more important than the appearance of the animal. Additional investigations of AAI treatment mechanisms are needed.