Warsaw is a city in and the county seat of Kosciusko County, Indiana, United States. Warsaw has a population of 15,804 as of the 2020 U.…Warsaw is a city in and the county seat of Kosciusko County, Indiana, United States. Warsaw has a population of 15,804 as of the 2020 U.S. Census. Warsaw also borders a smaller town, Winona Lake. Warsaw, named after the capital of Poland in tribute to Tadeusz Kościuszko, was platted on October 21, 1836. Warsaw's post office was established in 1837.