No Turn on Red Sign
MUTCD R10-11
Turning is prohibited while the traffic signal is red.
What it looks like
A vertical white rectangle with black legend reading NO TURN ON RED in capital letters.
When you'll see it
Posted at signalized intersections where turning against a red light is unsafe due to pedestrians, sightlines, or heavy cross traffic.
What to do
While the signal is red you must not turn; wait for a green light or arrow before making your turn.
Related signs
Source: Manual on Uniform Traffic Control Devices (MUTCD), 11th Edition — FHWA.