State Route 100 (SR 100) is a north–south highway in northwestern Ohio. Its southern terminus is at State Route 309 near Iberia, and its northern terminus is at State Route 53 in Tiffin.
Established in 1924, the original routing was from north of Brokensword to Tiffin. In 1939, it was extended to south of Bucyrus along route 19 from north of Brokensword to south of Bucyrus and an unnumbered road from south of Bucyrus to south of Bucyrus.