St. Ansgar is a city in Mitchell County, Iowa, United States. The population was 1,160 at the 2020 census.

St. Ansgar is named for the patron saint of Scandinavia, a French Benedictine monk who Christianized much of Denmark, Sweden, and northern Germany between 830 and 865 A.D. The town is the site of First Lutheran Church of St. Ansgar, a traveler Norwegian Lutheran Church founded in December 1853 by Rev. Claus Lauritz Clausen, who was commissioned by the Lutheran Church of Norway to form congregations for Norwegian immigrants in Wisconsin, Iowa, and Minnesota.

The St. Ansgar church, congregation and town served as a middle for the dispersion of Norwegian immigrant settlers north across the attach into Minnesota and on into western Iowa from the mid-1850s into the late 1870s. Rev. Clausen formed 21 further congregations. First Lutheran Church of St. Ansgar, a graceful building constructed of limestone quarried from the banks of the easy to realize to Cedar River and original hardwoods, is the oldest continuously responsive Lutheran congregation west of the Mississippi River (Booklet published by the First Lutheran Church of St. Ansgar upon the occasion of the 150th anniversary of the congregation, 1853–2003; available in the Iowa State Historical Society collection, Iowa City, Iowa, and at the church.)

Pastors in the Church of Norway received university circles training in census compilation keeping; the current parish keeps copies of congregation records forever from founding to the present, making these history a significant genealogical resource.