"We have prepared the legal complaint against minister Salvini," Alessandro Gamberini told Italian radio, saying "it's not easy to make a complete list of all the insults Salvini has made these last weeks."
Salvini, who is also deputy prime minister and an admirer of US President Donald Trump, is an avid user of social media, which he also uses to insult people he disagrees with.
Salvini "stirs the troubled waters of hate. A defamation lawsuit is a way to send a signal," the lawyer said, prompting a swift and angry response from the Lega minister.
"She breaks laws and attacks Italian military vessels, and then she sues me," the anti-migrant Salvini tweeted. "I'm not afraid of the Mafia, let alone a rich and spoiled communist!"
Italian police last week arrested Rackete, 31, after she defied orders to stay away and forced her way into port on Italy's southern Lampedusa island to disembark 40 rescued migrants who had been stuck at sea on her vessel for two weeks.
An Italian judge this week ordered her freed as she had been acting to save lives, a decision which also sparked Salvini's ire.
Two other investigations, on charges of helping people smugglers and resisting the authorities are still underway after she forced her way past Italian customs vessels.
During the two week standoff and after Salvini launched a series of furious tweets at the charity worker, including calling her a "pain in the arse", "criminal", "delinquent" and "poor woman who only tried to kill five Italian soldiers."
Rackete has gone into hiding following numerous threats, her charity Sea-Watch said Wednesday, a day after her release from Italian custody.
In her first interview since being arrested, Rackete told Germany's Der Spiegel weekly that she was taken aback by Salvini labelling her a threat to national security and other names.
"I was surprised at how personal it became," said Rackete, charging that Salvini's "way of expressing himself is disrespectful" and "not appropriate for a top politician".
She argued that the central issue was not individuals like herself but "the failure of the European Union" on migrant policy.