Right Turn Only Sign
MUTCD R3-5R
Traffic in this lane must turn right.
What it looks like
A white square with a black arrow curving to the right and the word ONLY, telling drivers the required movement for the lane.
When you'll see it
Posted at intersection approaches over or beside a dedicated turn lane, and often reinforced by an arrow painted on the pavement.
What to do
If you are in this lane you must turn right; get into a through lane beforehand if you intend to continue straight.
Related signs
Source: Manual on Uniform Traffic Control Devices (MUTCD), 11th Edition — FHWA.