Therapists’ and clients’ unplanned, spontaneous experiences in the time between therapy sessions have been defined as intersession experiences (ISE). This systematic literature review aims to inform clinical practice and future research by quality appraising and synthesizing empirical evidence relating to therapists’ ISE. Searches of electronic databases identified 1,499 articles; 6 met the selection criteria, with a further 6 articles identified through citation and hand searching, resulting in 12 articles in total: 6 were qualitative, 4 quantitative, and 2 were mixed-method. To assess methodological quality, an adapted appraisal tool was used. Quantitative articles were synthesized using a narrative approach. Metaethnography was used to synthesize qualitative articles. Reported frequency of ISE varied among therapists, which may be accounted for by factors that influence the onset of therapists’ ISE, such as experiencing difficulty in therapy. Therapists from different theoretical orientations experience and use ISE differently. Qualitative synthesis identified ISE as a process of discovery for therapists, which they subsequently use to plan and prepare for therapy or discuss with others. Clinical implications include the identification of therapists’ ISE as a common process of therapy, independent of theoretical orientation, that can potentially contribute to outcome. Future research should focus on strengthening the quality of research in this area, as well as exploring further aspects of ISE, particularly the possible interrelationship between therapist and client ISE.