Brathwaite's signing comes less than a week after Australia speedster Mitchell Starc confirmed he would also pull on the magenta again this summer.
The 32-year-old all-rounder will arrive in Australia before the start of the tournament and will join Tom Curran and James Vince as the Sixers' three international players to now have signed with the defending champions for this season.
Brathwaite played four games for the Sixers in BBL 7 after coming in as a replacement player, helping the side on a late season four-game win streak that left them two points shy of the semi-finals.
"I have very fond memories of Sydney," Brathwaite said from Pakistan where he is playing in the Pakistan Super League for Peshawar Zalmi as per the Sydney Sixers website.
"I think Sydney is easily becoming my favourite city. I have good memories of playing at the SCG in my second Test match and even better memories of playing BBL for Sixers. The aura of the ground and cricket in NSW with all the history there is special. I remember great times dancing at the games and the times in the dressing rooms with our team. It's a great organisation," he added.
Sixers head coach Greg Shipperd said the Sixers were looking forward to everything that the hulking Brathwaite brings to his cricket.
"We are excited about the prospect of being reunited with Carlos for this unique season 10 of BBL," Shipperd said.
"We were struggling three seasons ago and Carlos joined as a replacement player and we finished four from four and he contributed greatly to our rebalance. Carlos is a man who embraced our on and off field endeavours so we are keen to have him back in the fold and contributing his experience to our chase for consistency and another shot at a title," he added.
The Sixers will begin their title defence on December 10 against the Hobart Hurricanes in the first of three away fixtures in Tasmania.