Saha has been without a club for over a month after cutting short his contract with Sunderland but is adamant that he can help Lazio achieve their aim of securing Champions League football this season.
He told reporters: "I am very happy to be here at this big club. I have found a great group and working with them will be a real pleasure for me. I want to give my best.
"I am here to help the team during this injury and I'm happy to do it. I know on Saturday it will be a big game against Napoli and I am here to help. In life you must always try to give your best.
"I am a very competitive person, so the important thing is to always do better. This is a great team and a great country. I am happy to be here."
The 34-year-old has been signed primarily to help Lazio cope with the loss of top scorer Miroslav Klose, who has been ruled out for two months with a knee injury.