The couple, who met on the set of Black Swan, announced their engagement in 2010.
Reports of a secret marriage between the pair emerged in February this year, when the Star Wars actress was spotted wearing a wedding band at the Academy Awards.
However, sources close to the couple have told In Touch Weekly that Portman and Millepied have scheduled a wedding ceremony for August 5.
The source explained: "Natalie and Ben will marry in Big Sur. It's one of Natalie's favourite places - whenever she needs a break from LA, she comes here to escape."
Portman and Millepied's ceremony will apparently be intimate, limited to only their closest friends and family.
"It will be a very relaxed affair," the source adds. "They're not sending out invites. Natalie may not even buy a new dress!"
The couple are parents to 1-year-old son Aleph.