Background and Objectives: Remdesivir (RDV) is the first antiviral agent approved in Japan for the treatment of coronavirus disease 2019 (COVID-19). The aim of our study was to assess the efficacy and safety of RDV treatment in mildly to moderately ill patients with COVID-19. Materials and Methods: A single-center, retrospective study was performed in Fukuoka University Chikushi Hospital. Patients admitted to our hospital from June to October 2021 for RDV treatment against COVID-19 were enrolled. The primary end point was clinical status on days 10 and 14, using a 6-point ordinal scale ranging from death (category 6) to discharge (category 1). Adverse events were assessed and graded using the Japanese version of Common Terminology Criteria for Adverse Events (CTCAE) v5.0. Results: In total, 47 COVID-19 patients receiving RDV treatment were assessed during the study period. Thirty-four (72.3%) out of 47 patients required oxygen therapy. Out of these 34 patients, 30 (88.2%) showed a 2-point clinical improvement on day 14 after RDV was initiated. Serum alanine aminotransferase levels were elevated in three patients (6.4%) (CTCAE Grade 3) and neutropenia was detected in one patient (2.1%) out of the 47 patients. Conclusions: RDV may be highly effective, with good safety profiles, in patients with COVID-19 requiring oxygen therapy.