Background Many former inmates recidivate, resulting in high costs for societies worldwide. Evidence based treatment practices may not work in prisons, due to detainees’ lacking motivation, impaired well-being, and an unsafe group environment. One attempt to improve social group climate and well-being is the use of Prison-based Animal Programs (PAP). Using a quasi-experimental design, the aim of the current study is to examine the effectiveness of one such PAP in the Netherlands: Dutch Cell Dogs (DCD). Methods/Design Participants (N = 256) from 12 justice centers, including psychiatric, juvenile and adult facilities, will be recruited. Half of the sample (n = 128) will receive DCD training after voluntarily signing up (intervention group); The other half (n = 128) will be recruited to participate in the research and receive treatment-as-usual (TAU/Ccomparison group). Factors related to psychosocial functioning (e.g., self-esteem, empathy, self-control, life satisfaction, attention) and general therapeutic factors (i.e., therapeutic alliance, treatment motivation), expected to contribute to treatment success, will be assessed to measure the effectiveness of DCD. In addition, behavioral problems will be measured as well as recidivism rates. Questionnaires and neuropsychological tests will be employed to measure aforementioned outcome variables. Moreover, physiological data, based on heart rate and cortisol measures, will be collected to provide insight into the functioning of participants’ physiological stress response and to determine whether stress reduction occurs over time. Multimethod data collection will occur at pre-training (T1), at 1-month (halfway training/T2), at 2-months (end training/T3), and 6-months after the end of the training (follow up/T4). Discussion This is the first study to examine the effectiveness of a widely implemented PAP in the Netherlands. Challenges associated with conducting the proposed study are typical for practice based research in correctional settings (e.g., a demanding ...