State Road 18 (SR 18) in the U.S. State of Indiana is an east–west route in North Central Indiana running from the Illinois border in Benton County almost to the Ohio border, terminating at U.S. Route 27 (US 27)/State Road 67 (SR 67) in Jay County.

State Route 18 is mainly a 2-lane road with exception of 4-lane divided highway from Marion to Interstate 69 (I-69).

SR 18 heads east from the western terminus at the Illinois State line.

SR 18 heads east towards Brookston, passing through an intersection with U.S. Route 231 and an interchange with Interstate 65 (I-65).

When in downtown Delphi SR 25 leaves the concurrency on a northeast route as US 421/SR 18/SR 39 turn southeast.

SR 18 heads east towards Galveston passing through a concurrency with State Road 29.