Reja, who left his job as Hajduk Split on Tuesday, replace Davide Ballardini.
Lazio dropped into the relegation drop zone following their 1-0 defeat to Catania last weekend.
A statement from the club read: “SS Lazio announces that it has sacked coach Davide Ballardini and his staff.
“Coach Edoardo Reja has taken over the team.”
Reja signed a contract that will keep him at Lazio until June 2011.
