Diarra signed for Portsmouth on 17 January 2008. He was given his first start against Derby County, contributing to a 3–1 win at Fratton Park. He scored his first goal for the club in the following match, against FA Cup Fourth Round Championship opponents Plymouth Argyle, and his first Premier League goal two weeks later, a controversial late winner against Bolton Wanderers. Diarra went on to play in every minute of Portsmouth's remaining FA Cup matches en route to claiming the trophy in the final. He topped the season by being included in the France squad for UEFA Euro 2008; this and the FA Cup win arguably vindicated his decision to part company with Arsenal, Diarra himself saying that it had "more than justified my decision." He played in the 2008 Community Shield against Manchester United, missing in the penalty shootout, which was won by United. On 18 September 2008, he scored Pompey's first-ever goal in a major European competition against Vitória de Guimarães, his third and final goal for the club. The first red card of Diarra's Premier League career came on 28 September 2008 when he was booked twice against Tottenham Hotspur at Fratton Park.