Rensselaer County / rɛnsəˈlɪər / ren-sə-LEER is a county in the U.S. state of New York. As of the 2020 census, the population was 161,130. [ 2 ] Its county seat is Troy. [ 3 ] The county is named in honor of the family of Kiliaen van Rensselaer, the original Dutch owner of the land in the area.