| Thaddeus E. Cromley | |
| State Senate: | Ohio |
| District: | 10th |
| Term Start: | January 6, 1896 |
| Term End: | December 31, 1899 |
| Preceded: | Moses B. Earnhart |
| Succeeded: | Edward D. Howard |
| Alongside: | Nial R. Hysell (1896-98) John C. L. Pugh (1898-1900) |
| State House2: | Ohio |
| District2: | Pickaway |
| Preceded2: | Wesley Work |
| Succeeded2: | Daniel Haas |
| Term Start2: | January 2, 1888 |
| Term End2: | January 3, 1892 |
| Party: | Democratic |
| Birth Date: | 21 June 1843 |
| Birth Place: | Pickaway County, Ohio |
| Death Place: | Walnut Township, Pickaway County, Ohio |
| Restingplace: | Forest Cemetery Circleville, Ohio |
| Spouse: | Mary Rebecca Millar |
| Children: | eight |
| Alma Mater: | Kenyon College (A.B. 1865) (A.M. 1868) |
Thaddeus E. Cromley (21 June 1843 - 20 April 1921) was a Democratic politician from Ohio. He served in both houses of the Ohio Legislature, holding a leadership position in the Ohio State Senate during the nineteenth century.
Thaddeus Cromley descended from German immigrants named Grammlich, who settled in Pennsylvania in 1749. The name was eventually anglicized to Cromley. He was born on his father's Pickaway County, Ohio farm on June 21, 1843. His parents were William and Sarah E. (Staige) Cromley. He attended the local log schoolhouse, and he studied at Circleville Union School beginning at age 16. He entered Kenyon College in 1861 and graduated with an A.B. in 1865 and an A.M. three years later. He became a farmer and stock raiser.
Cromley served as a township trustee and member of the Board of Education. He was elected as a Democrat to the Ohio House of Representatives in 1887 by a plurality of 1,116 votes, and was re-elected in 1889 by a plurality of 1,090 votes. In 1890, he was the nominee for Ohio Secretary of State, but lost to Republican Daniel J. Ryan.
In 1892, Cromley was appointed by Governor McKinley as a trustee of the Boy's Industrial School in Fairfield County. He held that position until 1896. In 1895 he was elected to the Ohio State Senate from the tenth district (Franklin and Pickaway counties) with a plurality of 961 votes, and was re-elected in 1897 by 2,373 votes. In the latter term he was President Pro Tempore and chairman of the Committees on Finance and Rules.
In 1900, Cromley was elected to the State Board of Agriculture, and he was re-elected in 1905. He was married to Mary Rebecca Millar of Pickaway County on October 6, 1870. They had eight children. He was a member of the Knights of Pythias, Chi Phi and Phi Beta Kappa. Cromley died at his Walnut Township home on April 17, 1921. He was interred at Forest Cemetery in Circleville.