The throwback photo from 2004 shows the actress joined by Lisa Kudrow, Matthew Perry, Jennifer Aniston, David Schwimmer and Matt Le Blanc as they sit round a table full of food.
The actress, 55, captioned the photo: “‘The Last Supper’ before taping ‘The Last One’ on Jan 23, 2004. #tbt #friends.”
She also shared a snap of the script for the finale.
Aniston, 50, commented on the post with a string of crying emojis, while Kudrow wrote: “Awwww.”
Aniston joined Instagram back in October with a selfie featuring all five of her Friends co-stars. Guinness World Records confirmed shortly after that the star had broken the record for the fastest time to reach one million Instagram followers in five hours and 16 minutes.
Cox previously shared a photo from a trip the cast took to Las Vegas together before the show first aired.
She wrote: “The one where the six of us went to Vegas and no one knew we were F.R.I.E.N.D.S yet #tbt #beforeitaired #lovetheseguys."
The post comes amid rumours that an unscripted reunion special could be on the cards on streaming service HBO Max.
Kevin Reilly, the service’s chief content officer, recently told the Television Critics Association: “There’s interest all the way around, and yet we can’t get interest all aligned to push the button on it.
“Today, unfortunately, it’s still a maybe.”
Additional reporting by Press Association.