Hydroxychloroquine (HCQ) is known as one of the most fascinating synthetic antimalarial drugs during the last 50 years. It is currently among the most commonly employed medicines for the clinical treatment of rheumatic diseases, especially systemic lupus erythematosus and rheumatoid arthritis. In related mechanism studies, it has been found that HCQ possesses various immunomodulatory and anti-inflammatory activities. In addition, the effects of HCQ on anti-platelet, metabolic pathways, and antineoplasticity have also been disclosed in more recent studies. These significant findings on HCQ suggest the potential therapeutic applications of HCQ for treatment of many diseases, such as cancers, skin disease, antiphospholipid syndrome, etc. This review focuses on recent in vitro and clinical trials on its pharmacological mechanisms, therapeutic activities, and potential adverse effects.