<b><i>Background:</i></b> Low adherence to therapies in psoriasis decreases treatment outcomes and increases the total health care costs. In spite of the wide use of biologic agents, patients' adherence to these drugs has not been extensively investigated. <b><i>Objective:</i></b> The aim of this study is to measure adherence to the biologic drugs in a population of patients treated for psoriasis vulgaris using the medication possession ratio (MPR) index and to survey patients' attitudes to the treatment. <b><i>Methods:</i></b> This is a single-center study on 247 patients with psoriasis vulgaris treated with adalimumab (n = 113), etanercept (n = 39), and ustekinumab (n = 95). MPR calculation was calculated monthly based on the hospital records documenting the dispensing of biologics to the patients. Clinical data [Psoriasis Area and Severity Index (PASI), Dermatology Life Quality Index (DLQI), presence of psoriatic arthritis, concomitant treatment, and cause for treatment discontinuation] were obtained from the national database DERMBIO. Patients' attitudes and beliefs were measured using the Medication Adherence Rating Scale (MARS). <b><i>Results:</i></b> A total of 93.5% of all patients had an MPR ≥0.8, indicating very good adherence. MPR was independent of patients' age, gender, median PASI or DLQI score, concomitant diagnosis of psoriatic arthritis, or treatment with methotrexate. MPR for etanercept was slightly lower than that for ustekinumab and adalimumab. The MARS questionnaire documented satisfactory understanding of the treatment and its adverse effects, and positive attitudes to the treatment. <b><i>Conclusion:</i></b> Adherence to biologic therapies is very high in patients with psoriasis, which is consistent with a positive attitude to the treatment.