Background: The identification of factors associated with nonadherence to psychotherapy would allow a better prevention of this problem.Aim: To investigate factors associated with nonadherence in psychotherapy, its possible effect on outcomes, and reasons for therapy dropout.Method: Prospective analytic observational study of patients who initiated psychotherapy (N = 144). Sociodemographic, general clinical, treatment-related, subjective, psychological, psychopathologic, and outcome variables were evaluated at baseline and 1, 3, 6, and 12 months later. Objective nonadherence (dropout and irregularity), subjective nonadherence (poor patient engagement), and global nonadherence (combination of both) were analyzed.Results: Global nonadherence was 66%. Global nonadherence was associated with substance use or abuse (OR = 2.64) and younger age (OR = 0.97). Objective nonadherence was associated with active working status (OR = 4.11), younger age (OR = 1.04) and substance use or abuse (OR = 2.35). Subjective nonadherence was associated with worse insight in psychotherapy (OR = 0.95) and poor pharmacologic adherence (OR = 0.55). Contextual reasons (25.8%) were the most commonly reported cause of dropout. Time in psychotherapy was associated with outcome variables.Conclusions: Nonadherence to psychotherapy is frequent. To reduce nonadherence in psychotherapy, specific interventions for reducing substance use and abuse, measures aimed at facilitating access to Community Mental Health Units, and enhancing insight in psychotherapy should be implemented.