The modern SR 26 was designated on October 1, 1926 on a route that ran between Lafayette and Oakford, this route was previously known as SR 29. The modern route of SR 26 between the Indiana–Illinois state line and US 41 was added between 1927 and 1928, as SR 22. During 1930 a state road was proposed between US 41 and Lafayette and between US 31 and SR 9. Also at this time a state road was proposed from Upland east to the Ohio–Indiana state line. Between late 1930 and early 1932 the highway commission added SR 26 between US 31 and SR 9 to the state road system and the SR 22 designation west of US 41 was changed to SR 26. During this time frame SR 26 was added to the state road system between Hartford City and SR 67 and the modern route of SR 26 between the modern eastern end of SR 22 and Hartford City became part of SR 22. During 1932 the proposed route between Portland and the Ohio–Indiana state line was commissioned as part of SR 26 and the small north–south segment of modern SR 26, southeast of Upland, was commissioned as part of SR 221. SR 26 proposed to be extended east from SR 9 to SR 221 between 1936 and 1937. The road was extended from SR 9 to Hartford City between late 1938 and early 1939. The entire road was paved between 1966 and 1967.