The Sun says former Liverpool striker Cisse, 27, became Sunderland's fifth summer arrival and the club's highest-ever paid player on £50,000 a week.
Keane said: "He's a player we believe that people will say 'I didn't see that one coming'.
"I want to bring in players who we know can play in the Premier League.
"We're looking for two more defensive players and two attacking players."
Cisse could make his debut at Spurs on Saturday.