The former CS Sfaxien midfielder has penned a lucrative five-year contract with the English Premier League side - and arrives as David Moyes seventh signing of the transfer window.
Ndong, who has been capped 18 times with the Gabon national team, admitted the move to the Black Cats was a dream come true.
"I am very proud and happy to sign for Sunderland," he told the club's website. "This is a new adventure for me and to play in the Premier League is a dream come true.
"Maybe the Sunderland fans don't know me yet, but I promise that they will quickly discover that I will give everything for them and the club."