This study evaluates the impact of the COVID-19 pandemic on testing for common sexually transmitted infections. Specifically, changes are measured in chlamydia and gonorrhea testing and case detection among patients aged 14–49 years during the COVID-19 pandemic. U.S. chlamydia and gonorrhea testing and positivity were analyzed on the basis of >18.6 million tests (13.6 million tests for female patients and 4.7 million tests for male patients) performed by a national reference clinical laboratory from January 2019 through June 2020. Chlamydia and gonorrhea testing reached a nadir in early April 2020, with decreases (relative to the baseline level) of 59% for female patients and 63% for male patients. Declines in testing were strongly associated with increases in weekly positivity rates for chlamydia (R²=0.96) and gonorrhea (R²=0.85). From March 2020 through June 2020, an expected 27,659 (26.4%) chlamydia and 5,577 (16.5%) gonorrhea cases were potentially missed. The COVID-19 pandemic impacted routine sexually transmitted infection services, suggesting an increase in syndromic sexually transmitted infection testing and missed asymptomatic cases. Follow-up analyses will be needed to assess the long-term implications of missed screening opportunities. These findings should serve as a warning for the potential sexual and reproductive health implications that can be expected from the overall decline in testing and potential missed cases.