A man's reaction to an apparently racist woman at a US airport apparently caused onlookers to burst into applause, and has attracted hundreds of thousands of shares online.

Emmit Eclass Walker said he was approached by a woman at a Washington airport who assumed he was not a first class passenger.

He believes it was due to the colour of his skin.

"Excuse me, I believe you may be in the wrong place... this line is for priority boarding," she told him, according to his Facebook post.

Mr Walker asked if she meant she was a priority because she was travelling first class, to which she replied "yes".