The objective of this study is to assess the effectiveness of resistance training in patients with psoriatic arthritis (PsA). The study is a randomized controlled trial with 41 patients aged between 18 and 65 years with diagnosis of psoriatic arthritis (PsA). The patients were randomized into the following: intervention group (IG) and control group (CG). The IG underwent resistance exercise twice a week, for 12 weeks. The CG remained with the conventional drug therapy. The outcome measurements were the following: BASFI and HAQ-S for functional capacity, one maximum repetition test (1RM) for muscle strength, SF-36 questionnaire for general quality of life, and BASDAI and DAS-28 for disease activity. The evaluations were done by a blinded evaluator at baseline (T0) after 6 (T6) and 12 weeks (T12). At baseline, the groups were homogeneous regarding clinical and demographic characteristics. The IG significantly improved functional capacity measured by HAQ-S and disease activity measured by BASDAI, compared to CG, at week 12. Regarding quality of life, the IG improved the domains “pain” and “general health status” compared to CG (p < 0.05). There was improvement in muscular strength in almost all exercises in IG, except in the exercise for biceps. However, there were statistical differences between groups only on exercise “leg extension” in IG compared to CG. Resistance training is effective in improving functional capacity, disease activity, and quality of life of patients with psoriatic arthritis. The clinical improvements were not coupled to significant changes in muscular strength.