The 23-year-old former Arsenal and Birmingham player has moved to Fratton Park on loan until the end of the season.
He told the club's official website: "I'm looking to bring a lot to the club. I'm happy to be here and hopefully I can have a big impact and help the club stay in the Premier League.
"If we stay focused then I'm sure we can win some matches and get out of the relegation places.
"I've played in the Premier League before and it's great to be back.
"I can't wait to get out on the pitch and play my first game for Pompey - that's what I'm waiting for."