She was so nervous, she couldn't pick out what to wear, People magazine reported.
"I must have tried on like six outfits," Witherspoon, 35, said Friday on "The Ellen DeGeneres Show."
"I'm meeting a princess. You don't understand. You have to be wearing the right outfit and the whole thing. I don't know, maybe we can be friends."
Witherspoon said her husband, Jim Toth, teased her for being so excited.
"Whenever I am super excited about things, my husband calls it the double-barrel. That's when I smile so big I look like a child," she said.
"I would like to hang out," Witherspoon said of Middleton. "I'm not sure she wants to hang out with me. I could see that happening."