The former France striker - released by Sunderland on transfer deadline day - joins the Serie A club on a free transfer.
"I am very happy to have joined this great club," said Saha, who will wear the No 28 shirt, at a press conference on Wednesday evening.
"I trained with a great squad this afternoon. I am ready to play against Napoli at the weekend."
Lazio this week lost forward Miroslav Klose to a knee injury which will keep him out for up to two months.
And Saha, 34, is the chosen replacement despite a lean time at Sunderland, where he failed to score in five months.