Foster married Jane Denny McDowell (1829 - 1903) on July 22, 1850 in Trinity Episcopal Church, Pittsburgh. She was the daughter of Andrew N. McDowell, a Pittsburgh physician, and Jane Denny Porter. The attraction between Foster and McDowell remains a mystery: Jane was on the edge of the Foster circle of friends, and had no special musical talents or interests. She may have broken an engagement to another man to marry Foster. The couple's only child Marion, a daughter, was born on April 18, 1850.