State Route 51 (SR 51) is a northwest-southeast highway (signed north-south) in northwest Ohio.

The portion from the Maumee to its southern (eastern) terminus was signed north-south.

From its southern terminus, State Route 51 heads north to an interchange with the Ohio Turnpike before it enters Elmore.

After exiting Elmore, the route heads northwest through the village of Genoa and near the town of Millbury before entering Northwood.

The route interchanges with Interstate 280 before entering southwest Oregon and into Toledo.