The Biancocelesti announced the completion of the deal on Wednesday, with the centre-back joining for an undisclosed fee.
Acerbi, who spent five seasons with Sassuolo, arrives as a replacement for Stefan de Vrij after the Dutch defender joined Inter on a free transfer.
Simone Inzaghi guided Lazio to fifth last term, securing a berth in the Europa League group stage.
The Roman side were also a matter of minutes away from taking a spot in the Champions League in a final day showdown with Inter.
After leading 2-1 at the Stadio Olimpico, however, goals from Matias Vecino and Mauro Icardi saw Lazio beaten 3-2 in a late comeback, forcing them to settle for fifth and the Europa League.