Hamilton County

US 127 enters Ohio from the Kentucky via the Clay Wade Bailey Bridge, which also carries US 42 and US 25 north across the Ohio River into Cincinnati. Upon entering Ohio, US 25 ends, and US 127/US 42 continues through Downtown east for one block on Third Street, and then north one block on Central Avenue. At this point, the route becomes a one-way pair, with the northbound route using Fifth Street and Elm Street, and the southbound route using Sixth Street and Plum Street. Both streets intersect Central Parkway, and US 127 follows the roadway north while US 42 instead heads east on the parkway.

As US 127 continues north, it passes one block west of Washington Park and Findlay Market in Over-the-Rhine and marks the neighborhood's western boundary. Upon passing the eastern end of the Western Hills Viaduct, the roadway closely parallels Interstate 75 (I-75) for nearly two miles (3.2 km).

US 127 then turns onto the Ludlow Viaduct over Mill Creek near the junction of I-75 and I-74; it becomes Hamilton Avenue at its end. At this point, US 127 becomes a predominantly four-lane surface street. As Hamilton Avenue, US 127 is the major north–south artery and commercial corridor in Cincinnati's Northside and College Hill neighborhoods, and then the inner suburban cities of North College Hill and Mt. Healthy. There is an interchange with I-275 just before it crosses into Butler County.

Rest of Ohio

US 127 passes through Fairfield then reaches downtown Hamilton. In Hamilton, US 127 widens to a four-lane road just south of downtown before returning to a two-lane road at the northern edge of New Miami. From this point, US 127 roughly parallels Sevenmile Creek until it reaches Eaton.^[3]

Approximately seven miles (11 km) north of Eaton, US 127 intersects I-70 and US 40 (National Road) and proceeds to Greenville. Along this section of the route US 127 is a two-lane rural highway surrounded by farms. Approaching Greenville, the road widens to a four-lane limited access highway, serving as a bypass to the east of the city.

The road returns to a rural two-lane road and continues north to Celina, skirting the western edge of Grand Lake St. Marys and the associated state park. US 127 serves as Main Street in Celina, the county seat of Mercer County. Just north of Celina, the highway intersects US 33, which connects Ft. Wayne, Indiana, with Columbus, and then US 127 continues to Van Wert.

In Van Wert, US 127 is Washington Street and serves as the main road through the city's business district. The Lincoln Highway intersects US 127 one block west of the county courthouse. In the northern part of the city, US 127 has a partial cloverleaf interchange with US 30 and US 224. US 224 joins US 127 and continues north briefly before US 224 turns to the east and US 127 continues due north toward Paulding.

In Paulding, US 127 bends to the northwest to cross Flatrock Creek before reaching the courthouse and turning back to the north on Williams Street. North of the city, and just south of the Maumee River, the highway intersects US 24, a limited-access highway between Toledo, Ohio and Fort Wayne, at a diamond interchange before crossing the river.

US 127 intersects US 6, the Grand Army of the Republic Highway and with State Route 15 (SR 15) becomes Main Street in Bryan. At the northern city limit, SR 15 continues north to the Ohio Turnpike while US 127 begins a northeast route toward West Unity. After passing under the turnpike, US 127 has a brief concurrency with US 20 and then enters Michigan to the north in rural Lenawee County.^[3]