In the global strategy for polio eradication, environmental surveillance (ES) has been established worldwide to monitor polioviruses. In addition, nonpolio enteroviruses are simultaneously isolated from wastewater under this ES program. Hence, ES can be used to monitor enteroviruses in sewage to supplement clinical surveillance. In response to the coronavirus disease 2019 (COVID-19) pandemic, we also monitored severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in sewage using the polio ES system in Japan. Enterovirus and SARS-CoV-2 were detected in sewage from January 2019 to December 2021 and from August 2020 to November 2021, respectively. Enterovirus species such as echoviruses and coxsackieviruses were frequently detected by ES in 2019, indicating the circulation of these viruses. After the onset of the COVID-19 pandemic, sewage enterovirus detection and related patient reports were notably reduced in 2020 to 2021, suggesting changes in the hygiene behaviors of the human population in response to the pandemic. Our comparative experiment with a total of 520 reverse transcription-quantitative PCR (RT-qPCR) assays for SARS-CoV-2 detection demonstrated that the solid-based method had a significantly higher detection rate than that of the liquid-based method (24.6% and 15.9%, respectively). Moreover, the resulting RNA concentrations were correlated with the number of new COVID-19 cases (Spearman's r = 0.61). These findings indicate that the existing polio ES system can be effectively used for enterovirus and SARS-CoV-2 sewage monitoring using different procedures such as virus isolation and molecular-based detection. IMPORTANCE Long-term efforts are required to implement surveillance programs for the ongoing COVID-19 pandemic, and they will be required even in the postpandemic era. We adopted the existing polio environmental surveillance (ES) system for SARS-CoV-2 sewage monitoring in Japan as a practical and cost-effective approach. Moreover, the ES system routinely detects enteroviruses from wastewater and, therefore, can be used for enterovirus monitoring. The liquid fraction of the sewage sample is used for poliovirus and enterovirus detection, and the solid fraction can be used for SARS-CoV-2 RNA detection. The present study demonstrates how the existing ES system can be used for monitoring enteroviruses and SARS-CoV-2 in sewage.