The 25-year-old Senegalese centre-half's current contract was due to expire in June 2008 but he has been rewarded for his form as the Royals have impressed in their first season in the top flight.
Sonko joined Reading on a free transfer from Brentford in 2004 and was voted into the Professional Footballers' Association select XI for the Championship as the Berkshire club won promotion last season.
Reading chairman John Madejski said: "Ibrahima is a vital player for us and I am delighted that he has agreed to stay here until 2010.
"He is an exceptionally good defender and an equally important presence off the pitch, and he will play a central role in our continued development in the next few years."
Sonko has made 110 appearances for the club, and his decision to commit to a new deal has been hailed as a "massive signing" by director of football Nick Hammond.
He said: "Ibrahima has been an immense figure in our rise from the Championship into the Premiership, and to secure such a big asset who is widely recognised as an outstanding talent is great news.
"He has fantastic loyalty and commitment to the club, as demonstrated last season when he turned down the opportunity of playing in the African Cup of Nations to help our push for promotion, and that kind of attitude is the foundation of the success we've enjoyed in recent years."
Sporting Life