The defensive midfielder, who failed to win a regular first-team place at Arsenal, signed a three-and-half-year deal for an undisclosed fee, Portsmouth said on their Web site (www.portsmouthfc.co.uk).
"I read that Arsenal wanted me to stay but if you want to keep a player, you make him play," Diarra told French sports daily L'Equipe in an interview on Thursday.
"I'm very happy, I feel a lot better because I know I'm going to play (with Portsmouth)," the 22-year-old added.
Diarra, who had moved to Arsenal last year from Chelsea, said his main objective was to convince France coach Raymond Domenech to take him to Euro 2008.
"That's the goal," he said. "Raymond Domenech trusted me even when I was not playing and now, I'm going to play regularly. Some say that by joining Portsmouth, I'm making a mistake but I don't think so."
Diarra has won nine caps for France, all in 2007.
Portsmouth said Diarra would train with his new club for the first time on Friday ahead of Derby County's visit to Fratton Park on Saturday (1500 GMT).
"He's a fantastic midfield player", Redknapp said of his new signing. "In my opinion he's already one of the best midfield players in the Premier League and has the ability to be one of the best there is."
Portsmouth are ninth in the Premier League table, 17 points behind leaders Manchester United and second-placed Arsenal.