In the scene, the 67-year-old McKellen, who is portraying the tragic King Lear at the Courtyard Theater in Stratford-on-Avon, says: "Off, off you lendings (clothes)! Come unbutton here," and proceeds to tear off his clothes, the Daily Telegraph reported Saturday.
After surprising the first audience to see the show, the RSC will post notices warning the audience members of the nudity.
The RSC also said it was surprised that some people find McKellen's portrayal shocking.
A spokesman said: "The nudity takes place in the storm scene where Lear is going mad and is a very moving part in the play."
However, they will warn any school groups interested in attending well before the play opens on April 3.