| Frederick Olen Mercer | |
| Office: | Chief Judge of the United States District Court for the Southern District of Illinois |
| Term Start: | 1958 |
| Term End: | 1966 |
| Predecessor: | Charles Guy Briggle |
| Successor: | Omer Poos |
| Office1: | Judge of the United States District Court for the Southern District of Illinois |
| Term Start1: | June 19, 1956 |
| Term End1: | April 3, 1966 |
| Appointer1: | Dwight D. Eisenhower |
| Predecessor1: | J. Leroy Adair |
| Successor1: | Robert Dale Morgan |
| Birth Name: | Frederick Olen Mercer |
| Birth Date: | 11 March 1901 |
| Birth Place: | Vermont, Illinois |
| Education: | University of Illinois College of Law (LL.B.) |
Frederick Olen Mercer (March 11, 1901 – April 3, 1966) was a United States district judge of the United States District Court for the Southern District of Illinois.
Born in Vermont, Illinois, Mercer received a Bachelor of Laws from the University of Illinois College of Law in 1924. He was an assistant state's attorney of Fulton County, Illinois from 1924 to 1926. He was a county judge of Fulton County from 1926 to 1934. He was in private practice in Lewistown, Illinois from 1934 to 1940. He was in private practice in Canton, Illinois from 1940 to 1956.
Mercer was nominated by President Dwight D. Eisenhower on May 17, 1956, to a seat on the United States District Court for the Southern District of Illinois vacated by Judge J. Leroy Adair. He was confirmed by the United States Senate on June 13, 1956, and received his commission on June 19, 1956. He served as Chief Judge from 1958 to 1966. Mercer served in that capacity until his death on April 3, 1966.