Treatment resistant major depressive disorder (TR-MDD) is a severe disease, with very low remission rates. The resistance to pharmacotherapy leads to the search of non-pharmacological alternative approaches. Animal therapy has been used in patients with psychiatric conditions and the results have been promising. However, there have been no studies in TR-MDD patients with pet adoption. This study assessed the impact of TR-MDD patients adopting a pet. Eighty patients were suggested to adopt a pet, and 33 accepted the challenge. Other 33 patients constituted the control group (did not accept the suggestion of pet adoption and did not already have a pet). All patients maintained their usual pharmacotherapy. All participants were evaluated at baseline, 4, 8 and 12 weeks for depressive symptoms using HAMD17 and GAF. Results show that the pet group had an improvement in HAMD17 and GAF scores as well as higher response and remission rates compared to the control group, where no patient responded or remitted. Therefore, pets can be used as an effective adjuvant to pharmacotherapy with regular medical appointments.