Ecological Momentary Assessment (EMA) stands as a valuable method to capture real-time data on individuals' daily experiences and behaviors. In recent years, the utilization of EMA as a measurement method has substantially increased with the majority of studies emphasizing its clinical utility. However, a comprehensive overview of its use in psychotherapy research is lacking. This study addresses that gap by systematically reviewing EMA's application in psychotherapy research. In total, 168 studies met the inclusion criteria and were classified according to clinical utilization. Six areas of clinical EMA application were identified: prediction of therapy outcome (n = 8), prediction of psychopathology (n = 40), prediction of biopsychosocial states (n = 44), evaluation of therapy outcome (n = 21), acquisition of further clinical insights into specific disorders (n = 68) and adaptation of treatment processes (n = 18). Despite studies consistently highlighting EMA's potential in tailoring psychotherapeutic treatments, its limited use in this area warrants further research. Drawing from our findings, we discuss future research directions for the direct application of EMA in psychotherapeutic settings.