The actress told In Touch that Pattinson was not well when they filmed their sex scenes together.
Witherspoon said: "Rob possibly had the most hideous horrible cold of any co-star I've ever had to do a love scene with ever in my entire life.
"He was literally snorting and snotting through every second of it - and it was not appealing. I'm talking green, infectious, disgusting - I'm not kidding!"
She added: "I'm going to say it's a little bit of a downer. I was a little disappointed. It wasn't sexy."
Pattinson has claimed that Witherspoon is "a lot bawdier" and "a lot more raucous" than people might think.
> Robert Pattinson 'cried for elephant co-star'
Watch the Water For Elephants trailer below: