The R399 road is a short regional road in Ireland linking the N55 to the R393; all in County Longford.

Its route runs north of the Royal Canal. Abbeyshrule, situated on the canal and Abbeyshrule airport are just off the R399 to the south. The road is 5 km (3.1 mi) long.

• Roads in Ireland
• National primary road
• National secondary road

• Roads Act 1993 (Classification of Regional Roads) Order 2006 – Department of Transport