Harry Sanderson Day (May 16, 1871 – June 27, 1956) was a Republican politician in the U.S. state of Ohio who served as Ohio State Treasurer 1923–1927 and 1931–1937.

He also ran unsuccessfully for his party's nomination for Ohio Governor.

He was educated in the Fremont public schools, and worked for the railway mail service until 1891.

[1] He married Lola Garvin in 1901, and had a daughter named Mary Elizabeth.

He was unsuccessful in a run for the Republican nomination for Ohio Governor in 1926.