The 43-year-old, who is worth an estimated 80 million pounds, is reportedly refusing to marry the director, 41, until he signs the agreement, reported Contactmusic.
"Jen's thrilled to be getting married but she's leaving nothing to chance. She wants to make sure all her assets are protected, especially with her track record with men," a source said.
Aniston has dated musician John Mayer in the past and was married to actor Brad Pitt.
Aniston and Theroux recently got engaged after dating for more than a year.