The A4113 between Leintwardine and Knighton was closed in both directions around the junction with Reeves Lane, near Bucknell, on the border between Shropshire, Powys and Herefordshire.
The road was closed off by Herefordshire Council from about noon yesterday as the waters rose in the River Teme following the recent heavy rain.
The closure follows a report of a car being stranded in 18 inches of water on the A488 near Knighton at 10.30pm on Sunday, which had to be pushed to safety by firefighters from Clun.
A spokesman for Herefordshire Council said: "Always keep safe during flooding, always focus on the safety of yourself and your family.
"Do not underestimate the danger and damage that a flood can cause. Flood water can be very dangerous – six inches of fast-flowing water can knock you off your feet and two feet can sweep away a car."
Flood alerts remain in place across the area on the Upper Teme and River Corve, including at Leintwardine, Onibury and Ludlow.
See all current Shropshire flood warnings and alerts here