Objectives To assess the efficacy and safety of hydroxychloroquine (HCQ) plus standard-of-care (SOC) compared with SOC alone in adult patients with COVID-19. Design Multicenter, open-label, randomized controlled trial. Setting 16 government-designated COVID-19 treatment centers in China through 11 to 29 in February 2020. Participants 150 patients hospitalized with COVID-19. 75 patients were assigned to HCQ plus SOC and 75 were assigned to SOC alone (control group). Interventions HCQ was administrated with a loading dose of 1, 200 mg daily for three days followed by a maintained dose of 800 mg daily for the remaining days (total treatment duration: 2 or 3 weeks for mild/moderate or severe patients, respectively). Main outcome measures The primary endpoint was the 28-day negative conversion rate of SARS-CoV-2. The assessed secondary endpoints were negative conversion rate at day 4, 7, 10, 14 or 21, the improvement rate of clinical symptoms within 28-day, normalization of C-reactive protein and blood lymphocyte count within 28-day. Primary and secondary analysis was by intention to treat. Adverse events were assessed in the safety population. Results The overall 28-day negative conversion rate was not different between SOC plus HCQ and SOC group (Kaplan-Meier estimates 85.4% versus 81.3%, P=0.341). Negative conversion rate at day 4, 7, 10, 14 or 21 was also similar between the two groups. No different 28-day symptoms alleviation rate was observed between the two groups. A significant efficacy of HCQ on alleviating symptoms was observed when the confounding effects of anti-viral agents were removed in the post-hoc analysis (Hazard ratio, 8.83, 95%CI, 1.09 to 71.3). This was further supported by a significantly greater reduction of CRP (6.986 in SOC plus HCQ versus 2.723 in SOC, milligram/liter, P=0.045) conferred by the addition of HCQ, which also led to more rapid recovery of lymphopenia, albeit no statistical significance. Adverse events were found in 8.8% of SOC and 30% of HCQ recipients with two serious adverse events. The most common adverse event in the HCQ recipients was diarrhea (10%). Conclusions The administration of HCQ did not result in a higher negative conversion rate but more alleviation of clinical symptoms than SOC alone in patients hospitalized with COVID-19 without receiving antiviral treatment, possibly through anti-inflammatory effects. Adverse events were significantly increased in HCQ recipients but no apparently increase of serious adverse events. Trial registration ChiCTR2000029868.