The Yellow Sign is some sort of occult symbol. Its appearance appears to be related to The King in Yellow. Apparently in Yhtill seeing the yellow sign means that the King in Yellow is about to make an appearance.

The appearance of the Yellow Sign appears to have had a profound effect on some of the viewers of the play, especially Nathaniel Browne. These effects seem to include vivid dreams and even hallucinations.

A yellow sign, painted in blood, was used to open the gates between Leng and Earth.

In the play Carcosa, or The Queen and the Stranger, The Stranger wears the Yellow Sign, apparently in defiance of The King in Yellow. He reveals it, to great effect, as the climax of the first act.

In Robert W Chambers' story 'The Yellow Sign' a young artist finds a brooch emblazoned with the Sign, after which his life rapidly descends into madness, horror and death.

The Sign is embossed on the cover of Talbot Estus' copy of Le Roi en Jeune.

Early in the campaign the Sign was seen in many unexpected places:

• As part of a piece of graffiti, by Doctor Frederick Bartlett.
• On sheet music, in the dreams of Nathaniel Browne,
• In the explosion of a firework, again by Browne.