The Black Cats will have the option to make the deal permanent at the end of the season.
"John is relishing the challenge of making an impact at Sunderland," boss Steve Bruce told the club website.
Mensah added: "I'm very delighted to join Sunderland today, it's like a dream come true for me because it's not easy to play in England.
"I want to tell my fans that they should not expect magic from me but that I will be giving my best.
"I have a lot of experience from Italy and Europe, while I have played a lot of games for my country and competed in the World Cup and the African Nations Cup.
"Because of this it is not difficult for me to play in England. Now I hope I can do well here in Sunderland and I will listen and do whatever the coach tells me to do."
Mensah has also had spells in Italian football with Modena, Chievo and Genoa, as well as Rennes in France.
John is a solid, uncompromising defender who adds that extra steel that I believe is essential Sunderland manager Steve Bruce The defender was a key player of the Ghana squad - along with Chelsea's Michael Essien, and Sulley Muntari of Inter Milan - which reached the 2006 World Cup finals.
Bruce added: "I'm a huge admirer of John having seen him on a number of occasions. I'm delighted to have added him to our back line.
"He's a solid, uncompromising defender who adds that extra steel that I believe is essential to have in the top flight."
Mensah will now compete for a place in the Black Cats' rearguard with Anton Ferdinand, Danny Collins, Nyron Nosworthy and Paulo Da Silva.