This article is about the current B5123 in rural Flintshire.
For the original B5123 in Mold in the same county, see B5123 (Mold)
The B5123 provides a link between the A55 and the A541 via Halkyn Mountain in central Flintshire. The road was originally unclassified but gained its number in the 1940s.
The road starts at a T-junction with the A5026, underneath junction 32A of the A55, and almost immediately gains a 40mph limit, which it retains for most of its length. The road passes through Pentre Halkyn, and briefly features a short stretch of NSL before regaining a 40mph limit up through the woods to Halkyn. The road then opens out as it passes along the mountain, through Berth Ddu, featuring some fantastic views over the Dee on a clear day. Rhosesmor is then passed through, and it's back up to NSL, although thick hedges and tight corners means even 40mph might be an excessive speed to do here. The road then opens out again and heads fairly steeply down the hill to end on the A541.