Despite reports claiming the couple - who have a year-old son, Aleph, had already tied the knot earlier this year, friends say the nuptials will take place on August 5, reported Femalefirst.
"Natalie and Ben will marry in Big Sur. It's one of Natalie's favourite places - whenever she needs a break from LA she goes there," the couple's friend said.
"This wedding is for their closest friends and family. It will be a very relaxed affair. They're not sending out invites. Natalie may not even buy a new dress!," the friend added.
Portman met choreographer Millepied on the set of "Black Swan" in 2009 and they got engaged in December 2010.
When she won the Best Actress Oscar for the film in 2011, she paid tribute to Millepied, calling him her "beautiful love" and thanked him for giving her the "most important role of my life".
She gave birth to their first child in June 2011.