Past Cincinnati, the route resumes along the Ohio River Trail, concurrent with USBR 25, to the Kentucky state line at Aberdeen.

[6][7] Across the Ohio River from Aberdeen in Maysville, USBR 21 continues along the Bluegrass Tour route to the Tennessee state line near Middlesboro.

USBR 21 runs from Cumberland Gap National Historical Park to Chattanooga.

[2] USBR 21 follows the Silver Comet Trail for 52 miles (84 km) from Cedartown to Smyrna.

[1] The American Association of State Highway and Transportation Officials (AASHTO) approved the southern segment of USBR 21 in Georgia in 2015[5] as the first U.S.