This retrospective study looked at the relation between SARS-CoV-2 RNAemia and COVID-19 critical condition development and mortality among 92 adult COVID-19 patients admitted to the Yokohama Municipal Citizen’s Hospital and National Institute of Infectious Diseases in Japan. The association between RNAemia and the on-admission conditions was assessed using 4C Mortality, CURB-65, and A-DROP scores. Of the 92 patients, 14 (15.2%) were in the RNAemia group and 78 (84.4%) were in the negative group. Among the RNAemia group, participants were older, had higher lactated dehydrogenase (p<0.001), C-reactive protein (p<0.001), and D-dimer (p=0.021), but had lower absolute lymphocyte count (p=0.025). Additionally, the 4C Mortality (p < 0.001), CURB-65 (p = 0.001), and A-DROP (p < 0.001) scores were higher in the RNAemia group than in the negative group. The RNAemia group had 12 (66.7%) of the critical cases, six (66.7%) of the fatal cases, one (4.0%) of the severe cases, and one (2.9%) of the moderate cases. The RNAemia group showed a strong association with higher mortality (OR, 18.75; 95% CI, 3.92–89.76; AUC, 0.7851; p < 0.001) and critical condition (OR, 72.00; 95% CI, 12.98–399.29; AUC, 0.8198; p < 0.001). These findings suggest that SARS-CoV-2 RNAemia is a strong predictive marker of mortality and critical condition development in COVID-19 cases.