Xeljanz ® (tofacitinib) is an oral small-molecule inhibitor that reversibly inhibits Janus-activated kinase (JAK)-dependent cytokine signaling, thus reducing inflammation. As a result of these mechanisms, effects on the immune system such as a moderate decrease in the total lymphocyte count, a dose-dependent decrease in natural killer (NK) cell count, and an increase in B-cell count have been observed. Therefore, tofacitinib provides an innovative approach to modulating the immune and inflammatory responses in patients with rheumatoid arthritis (RA), which is especially important in individuals who do not respond to tumor necrosis factor inhibitors or show a loss of response over time. The aim of this article was to review studies on the pharmacology, mode of action, pharmacokinetics, efficacy, and safety of tofacitinib in patients with RA. Tofacitinib has been shown to reduce symptoms of RA and improve the quality of life in the analyzed groups of patients. Moreover, it showed high efficacy and an acceptable safety profile in Phase III randomized clinical trials on RA and was the first JAK inhibitor approved by the US Food and Drug Administration (FDA) and European Medicines Agency (EMA) in the RA therapy, thus providing a useful alternative treatment strategy. Randomized controlled studies revealed a significant benefit over placebo in efficacy outcomes (American College of Rheumatology [ACR] 20 and ACR50 response rates); accordingly, clinically meaningful improvements in patient-related outcomes compared with placebo have been reported. The safety profile seems acceptable, although some severe adverse effects have been observed, including serious infections, opportunistic infections (including tuberculosis and herpes zoster), malignancies, and cardiovascular events, which require strict monitoring irrespective of the duration of tofacitinib administration. As an oral drug, tofacitinib offers an alternative to subcutaneous or intravenous biologic drugs and should be recognized as a more convenient way of drug administration.