Background: This study aimed to investigate the impact of preceding respiratory viral infections (RVI) on the clinical severity of pneumococcal pneumonia patients. Methods: A retrospective observational study was conducted at a university hospital from January 2009 to March 2013. Study subjects included adults (aged ≥18 years) with pneumococcal pneumonia who had undergone laboratory tests for RVI. Multivariate logistic regression analysis was performed to identify risk factors associated with severe pneumococcal pneumonia, defined as severity with the Pneumonia Severity Index (PSI) score ≥91. Results: In total, 191 patients with pneumococcal pneumonia were included for analysis and stratified into 2 groups: the severe group with a PSI score ≥91 (n = 99) and the non-severe group with a PSI score <91 (n = 92). Preceding RVIs were detected in 48 patients, including influenza A virus (n = 20), influenza B virus (n = 4), parainfluenza viruses (n = 5), metapneumovirus (n = 4), rhinovirus (n = 4), respiratory syncytial viruses (n = 6), coronaviruses (n = 2), and mixed viral infections (n = 3). In the multivariate logistic regression analysis, preceding RVIs (odds ratio [OR], 2·49; 95% confidence interval [CI], 1·10-5·60), male sex (OR, 2·58; 95% CI, 1·24-5·38), old age (OR, 2·92; 95% CI, 1·37-6·24), hypoalbuminemia (OR, 3·26; 95% CI, 1·56-6·84)], and azotemia (OR, 2·24; 95% CI, 1·08-4·67) were significantly associated with severe pneumococcal pneumonia. Conclusion: This study suggests that preceding RVIs might be one of the risk factors affecting the clinical severity of pneumococcal pneumonia.