He told Ellen: “I save your ones for special occasions. I was doing the last ‘Twilight’ movie and I only had one pair left and they were those ones.
“We were doing this big fight scene and for some reason, I think the shape of my body, my back is way too long or something, my ass is constantly hanging out.”
He joked producers on the movie were so concerned about the display they called up his agent.
Robert added: “I genuinely think I have a few too many vertebrae or something. We were doing this big fight and every shot has these Ellen underpants. It came to a point where producers are calling up by agent and going, ‘I don’t know what he is up to, but I don’t like it.’
However, the 24-year-old star has a reason for always being down to his last pair of underpants – he does not know how to wash them.
He confessed: “I don’t know how to use a washing machine.”
Robert has previously revealed he likes to wear the same clothes on a daily basis because he finds shopping a difficult experience.
He said: “I like kind of s***ty clothes most of the time. I like second hand. That's another thing. I can't really go into second hand stores to get new clothes any more so I end up wearing the same thing every day.
“I wish I could shop in vintage stores the whole time. But because a lot of paparazzi follow me around, it can be difficult.”