The B121 was a short-lived road in east County Tyrone. Originally unclassified, the road gained its number in the mid-1920s but had been declassified again within 20 years.
The road started on the B43 by the ancient High Cross in the centre of Donaghmore and headed southwest out of town, following a similar course to the River Torrent. It wasn't long before the road reached Castlecaulfield, where it ran through the western side of town. After passing the grounds of Parkanaur House the road ended at a T-junction on the now-bypassed original line of the A4.