The Italian electoral law of 2017, colloquially known by the nickname Rosatellum bis or simply Rosatellum after Ettore Rosato, the Democratic Party (PD) leader in the Chamber of Deputies who first proposed the new law, is a parallel voting system, which acts as a mixed electoral system, with 37% of seats allocated using a first-past-the-post electoral system and 63% using a proportional method, with one round of voting. The Chamber and Senate of the Republic did not differ in the way they allocated the proportional seats, both using the largest remainder method of allocating seats.
The new electoral law was supported by the PD and its government ally Popular Alternative but also by the opposition parties Forza Italia, Lega Nord, and Liberal Popular Alliance. Despite many protests from the Five Star Movement, the Democratic and Progressive Movement, Italian Left, and Brothers of Italy, the electoral law was approved on 12 October 2017 by the Chamber of Deputies with 375 votes in favor and 215 against, and on 26 October 2017 by the Senate with 214 votes against 61.
The law regulates the election of the Chamber and Senate, replacing Porcellum of 2005 and Italicum of 2015, both modified by the Constitutional Court of Italy after judging them partly unconstitutional.
As a consequence of the result of the 2016 Italian constitutional referendum, in which the Senate reform was rejected, the two chambers of the Italian Parliament ended up with two different electoral laws, which lacked uniformity. As a matter of fact, the electoral law for the Chamber of Deputies passed by Matteo Renzi's government, the so-called Italicum, based on a strong winner-take-all principle, was still in effect, whereas in the Senate a pure proportional system (the remnants of the so-called Porcellum, the electoral law approved by the cabinet of Silvio Berlusconi in 2005 and then declared extensively unconstitutional by the Constitutional Court in January 2014) was in force.