The episode will air on the HBO Max streaming service, which launches in May. A date is yet to be announced.
Jennifer Aniston, Courteney Cox, Lisa Kudrow, Matt LeBlanc, Matthew Perry and David Schwimmer will all appear.
Perry posted on Instagram: “It’s happening” with a photo of the cast from the 1990s. The rest of the cast then started sharing the same post.
Friends aired from 1994 until 2004. The final show was watched by 52.5 million viewers in the US, making it the most watched TV episode of the 2000s.
The show has since picked up legions of younger fans through Netflix.
It was the UK’s favourite streaming show and Netflix’s second most popular show in the US in 2018.
Rumours of a reunion intensified after Aniston posted a photo of the cast together on Instagram in October.
HBO Max has now secured rights to the show’s back catalogue for $425m (£339m).
“Guess you could call this the one where they all got back together – we are reuniting with David, Jennifer, Courteney, Matt, Lisa and Matthew for an HBO Max special that will be programmed alongside the entire Friends Library,” said Kevin Reilly, chief content officer for the channel.
He said the reunion special will capture the spirit of a time when “friends – and audiences – gathered together in real time”.
The cast will be involved in producing the episode.
According to Variety, each actor is expected to receive $2.5m (£1.9m) for taking part in the special, which will be available when HBO Max – a new subscription streaming service – launches.
Fans of the show reacted with glee to the announced on social media.
The entire friends cast signed on to do a reunion so I guess 2020 isn’t that bad after all.— Sarah Jenkins (@sarahjenkinsxo) February 21, 2020
𝙎𝙏𝙊𝙋. 𝙀𝙑𝙀𝙍𝙔𝙏𝙃𝙄𝙉𝙂.@hbomax have just confirmed that the entire @FriendsTV cast are going to reunite for an exclusive #FriendsReunion special when the streaming service launches in May. pic.twitter.com/qou07A8hIe— Omid Scobie (@scobie) February 21, 2020