This paper evaluates the effectiveness of 8-sessions of cognitive analytic therapy (CAT) for obsessive morbid jealousy (OMJ). The evaluation method was a mixed-methods A/B with follow-up single-case design. Ideographic jealousy measures were collected throughout baseline (2 sessions), treatment (6 sessions) and follow-up phases (one session) creating a 160-day time series. Nomothetic measures were completed at assessment, end of treatment and at follow-up. A structured qualitative interview was completed. Significant reductions to idiographic jealousy measures occurred during the treatment phase, with these gains maintained over follow-up. The intensity of the jealousy shifted from moderate to mild and this change was attributed to CAT. Methodological issues and future directions for the treatment of OMJ are discussed.