FIT-039 is a selective, ATP-competitive and orally active CDK9 inhibitor with an IC50 of 5.8 μM for CKD9/cyclin T1. FIT-039 does not inhibit other CDKs and other kinases. FIT-039 inhibits replication of HSV-1 (IC50 of 0.69 μM), HSV-2, human adenovirus, and human CMV. FIT-039 is a promising antiviral agent for inhibiting drug-resistant HSVs and other DNA viruses.