Jay County is a county in the U.S. state of Indiana. As of 2020, the population was 20,478. The county seat is Portland. The Indiana State Legislature passed an omnibus county bill on February 7, 1835 that authorized the creation of thirteen counties in northeast Indiana, including Jay - the only county in the United States named for John Jay, co-aut…Jay County is a county in the U.S. state of Indiana. As of 2020, the population was 20,478. The county seat is Portland. The Indiana State Legislature passed an omnibus county bill on February 7, 1835 that authorized the creation of thirteen counties in northeast Indiana, including Jay - the only county in the United States named for John Jay, co-author of The Federalist Papers, Secretary of Foreign Affairs under the Articles of Confederation, and first Chief Justice of the United States. John Jay had died in 1829.