State Route 424 is an east–west state highway in the northwestern portion of the U.S. state of Ohio. Its western terminus is at the Defiance County/Henry County line, and its eastern terminus is at U.S. Route 24 near Napoleon.

The State Route 424 designation is being removed and the road transferred to county engineer departments and cities as the U.S. 24 Fort-to-Port project between Fort Wayne, Indiana, and Toledo progresses. The westernmost portion in Defiance County was turned over in November, 2008, with more portions to follow.

Route 424 was the former routing of US-24 between Defiance and Napoleon before US-24 was rerouted onto its present alignment to the north of both cities in 1964.

Route 424C is a 0.17-mile connector that runs from SR 424 to US 24 east of Napoleon. SR 424 runs east until it intersects US 6 which then runs north and intersects US 24 just north of the SR 424 and US 6 intersection. Route 424C runs north from SR 424 as Township Road 10 and connects SR 424 with US 24 at the eastern terminus of SR 424. Route 424C connects to US 24 just east of the US 24 and US 6 intersection.