The purpose of this study was to develop a brief Functional Analytic Psychotherapy (FAP) protocol that will facilitate reliable implementation of FAP interventions, thus supporting research on FAP process and outcome. The treatment was a four-session individual therapy for clients who were interested in improving their relationship with their romantic partner. Data were collected from both the client and their partners. The treatment development process was conducted across two clinical case series. The first case series (n = 7) provided preliminary evidence of the feasibility and acceptability of the treatment, but rates of FAP interventions were low. Subsequently the treatment protocol and training and supervision procedures were revised. The second clinical case series (n = 6), based on the revised protocol, produced significantly higher rates of FAP interventions and further evidence of the acceptability and feasibility of the treatment. The study provides preliminary evidence that FAP interventions may be reliably implemented in the context of a brief, structured treatment.