As Italy heads towards elections, which must take place before May next year, Grasso said his Free and Equal group will promote reducing social inequality.
The group is mainly made up of ex members of the Democratic Party, who quit due to hostility towards leader Matteo Renzi. Renzi, who is hoping to make a comeback in the elections, was forced to resign as prime minister last December over a failed referendum on constitutional reform.
Grasso, a former national anti-mafia prosecutor who entered the senate in 2013, left the Democratic Party, which is the ruling party in the current coalition government, in October.
He dismissed the notion that voting for Free and Equal would mean wasting a vote on a small party while compounding the issues for the left, which is up against the insurgent Five Star Movement and a right-wing coalition forged by three-time prime minister Silvio Berlusconi in the leadership race.
The Five Star Movement is slightly ahead of the Democratic Party in opinion polls, with the most recent survey positioning the parties at 27.3 percent and 24.3 percent, respectively. Berlusconi's Forza Italia is in third place, with 15.3 percent, while the far-right Northern League is in fourth, with 14.3 percent.