Background: Clients’ adverse reactions 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. Method: The Positive and Negative Experiences of Psychotherapy questionnaire (PNEP) was developed based on pilot data, a literature review, and two existing scales for measuring primarily adverse experiences during psychotherapy. Mental health care patients (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 two-week interval in between. Results: The positive and negative experiences subscales of the PNEP were found to possess excellent internal consistencies (αs ≥ .90). The PNEP test-retest reliability was .93 for the positive experiences subscale, and .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. Conclusions: Ideally, psychological interventions are evaluated for risks and benefits to determine safety and success. The PNEP might be a promising instrument for achieving this goal, although replication and extension of current findings are needed.