Jennifer Aniston & Justin Theroux -- Secret Mansion Used To Pull Off Surprise Wedding

Jennifer Aniston and Justin Theroux pulled off the surprise wedding of the century by using an adjacent mansion as a decoy.

We’re told the key to keeping the wedding under wraps was an empty $10.7 million dollar property adjacent to her Bel-Air spread.

The home, located directly to the south, became a staging area ... the wedding decorations, tables and various supplies were all there, ready to be moved to Aniston's main house at the last minute.

Aniston has been renovating her property, and improvements included a paved pathway beneath a bed of trees between the 2 homes.

Trucks would pull into the neighboring driveway and funnel the wedding supplies into Aniston’s backyard.

TMZ broke the story ... the couple threw guests off by telling them they were attending Theroux's 44th surprise birthday bash... smart considering with surprise parties no one talks and everyone shows up on time.

Event staff who spent the past week setting up were also left in the dark about the wedding and not allowed to park near the property or bring their cell phones onsite.