The official news of Nzonzi's move, for an initial fee of ?3.5million which could eventually rise to £5million, followed the arrivals of fellow midfielders Charlie Adam and Maurice Edu from Liverpool and Rangers respectively.
Adam and Edu have been given the all-clear to feature in Saturday's Barclays Premier League clash with Wigan at the DW Stadium.
France Under-21 international Nzonzi told www.stokecityfc.com: "I am very happy to be here and I would like to thank Stoke for giving me this chance because I felt that I was ready for a move and a new challenge.
"There is strong competition for places here at Stoke, but that is part of the game and that can only help to make me an even better player.
"I heard that the club was interested in me a couple of weeks ago, although I think they have been looking at me for a long time."
Nzonzi began his career in France with Amiens before joining Blackburn in the summer of 2009, and was named Rovers' player of the year in his first season.
Stoke boss Tony Pulis said: "Steven is an outstanding young prospect. He has made a big impact in the Premier League since he arrived at Blackburn three years ago and I am sure he will prove to be an excellent addition to our squad. He is the type of player who will endear himself to our supporters."
Scotland international Adam has signed for an undisclosed fee - believed to be in the region of ?4million - just a year after Kenny Dalglish signed him for Liverpool from Blackpool.
Edu had his transfer rubber-stamped on Friday after Stoke had confirmed last Saturday he had agreed personal terms on a three-year deal.
Source: PA
Source: PA