Clinical data networks that leverage large volumes of data in electronic health records (EHRs) are significant resources for research on coronavirus disease 2019 (COVID-19). Data harmonization is a key challenge in seamless use of multisite EHRs for COVID-19 research . We developed a COVID-19 application ontology in the national Accrual to Clinical Trials (ACT) network that enables harmonization of data elements that that are critical to COVID-19 research . The ontology contains over 50,000 concepts in the domains of diagnosis, procedures, medications, and laboratory tests . In particular, it has computational phenotypes to characterize the course of illness and outcomes, derived terms, and harmonized value sets for SARS-CoV-2 laboratory tests . The ontology was deployed and validated on the ACT COVID-19 network that consists of nine academic health centers with data on 14.5M patients . This ontology, which is freely available to the entire research community on GitHub at https: //github.com/shyamvis/ACT-COVID-Ontology, will be useful for harmonizing EHRs for COVID-19 research beyond the ACT network.