Key Points Question Is the COVID-19 pandemic associated with the stage at which gastrointestinal cancer is diagnosed in Japan? Findings In this cohort study of 5167 patients, significant decreases were observed for the diagnosis of stage I gastric cancer and stage 0 to II colorectal cancer, whereas a significant increase was observed for the diagnosis of stage III colorectal cancer. Meaning These findings suggest that during the COVID-19 pandemic, there may have been fewer cases of screening-detected gastrointestinal cancer, and colorectal cancer may have been diagnosed at more advanced stages.