BackgroundClients' adverse experiences during psychotherapy are rarely monitored in clinical practice or research trials. One obstacle here is the lack of a measure to gauge both positive and negative experiences during psychotherapy. We developed and evaluated a new instrument for measuring such experiences.MethodThe Positive and Negative Experiences of Psychotherapy (PNEP) questionnaire was developed based on pilot data, a literature review, and two existing scales for measuring primarily adverse experiences during psychotherapy. Mental healthcare clients (N = 200) anonymously completed and evaluated the PNEP. Subsequently, a sample of professionals (N = 34) who underwent psychotherapy in the context of their training filled in the PNEP twice, with a 2‐week interval in between.ResultsThe positive and negative experiences subscales of the PNEP were found to possess excellent internal consistencies (αs ≥ 0.90). The PNEP test–retest reliability was 0.93 for the positive experiences subscale and 0.78 for the negative experiences subscale. For the positive subscale, four factors were extracted: symptom reduction and positive well‐being, high quality of therapy and therapeutic relation, personal growth and acceptance and interpersonal functioning. For the negative subscale, exploratory factor analysis suggested a three‐factor solution: escalation of symptoms and emotional distress, low quality of therapy and therapeutic relation and (self‐)stigmatization and dependency. Participants related a median of 13 positive and six negative therapy experiences to their most recent treatment. The most frequently endorsed negative experiences were having more negative thoughts and memories, feeling emotionally overwhelmed and an increase in stress due to the therapy. A minority of participants (10.5%) reported no negative treatment experiences.ConclusionsIn the field of psychotherapy, the evaluation of risks and benefits is crucial for assessing safety and effectiveness. The PNEP could be a promising instrument for achieving this objective, although further research is needed to replicate and expand upon the current findings.