Montgomery Oliver Koelsch (March 5, 1912 – September 1, 1992) was a United States federal judge. He died in Seattle, Washington.
Born in Boise, Idaho, Koelsch received a B.A. in History from the University of Washington in 1932 and an LL.B. from the University of Washington School of Law in 1935. He was an attorney in private practice in Boise, Idaho from 1936 to 1950 and served as Assistant Prosecutor for Ada County, Idaho from 1939 to 1945. He then served as a Judge for the Third Judicial District, Idaho from 1951 to 1959.
Koelsch was nominated to the United States Court of Appeals for the Ninth Circuit by President Dwight D. Eisenhower on September 12, 1959, to a seat vacated by James Alger Fee. He was confirmed as a federal judge by the United States Senate on September 14, 1959 and received his commission on September 23, 1959. He assumed senior status on January 31, 1976 and served in that capacity until his death on September 1, 1992.