The stars began dating seven years ago and now have six children together, but only recently announced their engagement, saying marriage is important to their young sons and daughters.
The Daily Mail newspaper in London said guests, including Pitt's parents and the jeweler who made Jolie's engagement ring, have arrived at Chateau Miraval in the southern countryside.
A reception hall has been built for the occasion next to a private chapel on the estate, the British newspaper said.
"Brad flew in to oversee preparations but they're all here now," a source told Britain's The Sun newspaper. "They say it's for Jane and Bill [Pitt,] but some think this could be it. Even Angelina's private jeweler is here. ... There's a real buzz around the estate."