Poestenkill is a town in Rensselaer County, New York, United States. The population was 4,530 at the 2010 census. The town is named after the Poesten Kill, an important stream in the area. The words "poesten kil" (with only one l) are traditionally supposed to signify "foaming water" or "foaming creek" in Dutch. While kil indeed means water or creek in Dutch, the etymology of foam for poest is not supported by Dutch dictionaries. It seems more likely that the creek was named after the nickname Poest for the 17th century local farmer and miller Jan Barentsen Wemp, who had a pimple (poest) or had difficulty breathing (poesten = to breathe); he owned a farm and a mill on the Poestenkil creek. Among the earliest settlers in the town was Archelaus Lynd. He leased 300 acres of land from the Van Rensselaers and made his first clearing in the area of Hillside Cemetery, which was known as The Lynd Cemetery until about 1900. The Whyland and Ives families were other early settlers. The town is southeast of Troy and is centrally located in the county.
The first school in the area was established in or soon after 1788 on land set aside for that purpose by Stephen Van Rensselaer III. Five men from Poestenkill served in the War of 1812, all joining the Eddy expedition.
The first regular church in Poestenkill was the First Baptist church, organized in 1814 and located in East Poestenkill.
During the American Civil War, many of Poestenkill's men served with the Union's forces; twelve who died in action were Daniel M. Horton, Philip Amidon, Daniel Morrison, John Wagoner, George Bradt, Dexter Randall, William H. H. Wood, Martin Larabee, Willard Bailey, W. L. Robbins, George Simmons, and William H. Mason.
Future president James A. Garfield considered a job as high school principal in Poestenkill in 1856, though lost it to another applicant.
Poestenkill celebrated its sesquicentennial (150th anniversary) in 1998, with several events including a Revolutionary War reenactment, antique car show, a parade, and a historical display. A postcard was also produced for the town at the time.