Italian media had reported on Tuesday the 65-year-old Sarri had decided to step down after five losses in Lazio's last six matches.
Sarri was in his third season in charge of Lazio and had a contract until the end of next season.
"Lazio announces that Maurizio Sarri has resigned as coach," the team said.
"The club thanks the coach for the achievements reached and for the work done, wishing him the best luck humanely and professionally.
"At the same time, the club announces that the coaching duties have been given to Giovanni Martusciello."
Martusciello was Sarri's assistant at Empoli, Juventus and Lazio. He was Empoli's head coach in 2016-17, when the Tuscan club was relegated on the final day of the season.
Martusciello will make his debut at nearby Frosinone on Saturday.
Sarri steered Lazio to second place in the Italian league last year, albeit 16 points behind runaway Serie A champions Napoli, but the capital club has disappointed this campaign.
A 2-1 defeat to Udinese on Monday was Lazio's 12th in the league this season and left them 11 points behind fourth-placed Bologna.
Lazio memorably beat Bayern Munich 1-0 in the first leg of the Champions League round of 16 last month but went on to lose 3-0 in Germany and were eliminated.
They also lost 3-0 to eventual winners Inter Milan in the semi-finals of the Super Cup in January. Lazio are through to the Italian Cup semi-finals, where they will face Sarri's former club Juventus next month.
Sarri was fired by Juventus in 2020 despite leading the Bianconeri to a ninth successive title in his one season in charge.
Sarri, a former banker, also coached Chelsea and Napoli.
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