The Ohio State League was a minor league baseball league that operated in numerous seasons between 1887 and 1947, predominantly as a Class D level league. League franchises were based in Indiana, Kentucky, Michigan, Ohio and West Virginia.

The first "Ohio State League" was a baseball league in 1887 before changing names to the Tri-State League. The Kalamazoo Kazoos won the first league championship. The league played in the 1889 and 1898 seasons.

The first 20th century Ohio State League started in 1908 as a class D loop and ran through 1916.

Another Ohio State League was formed in 1936 and ran through 1947 except for a break in '42-'43 due to World War II. In 1945, the OSL was one of just four class D circuits in operation, yet it was one of the first of dozens of leagues to collapse after World War II.

The class D Ohio–Indiana League was formed in 1948 with eight teams, with seven of the eight cities coming from the Ohio State League. Six teams made their home in Ohio and the remaining two where from Indiana. This league lasted through 1951.

The Portsmouth Cobblers won four Ohio State League championships.

Baseball Hall of Fame member Ed Delahanty played for the 1887 Mansfield team.

1887 Ohio State League