Mr Fitzmaurice will join his Independence Party colleagues Michael Collins and Richard O’Donoghue, who announced the formation of the new party last November.
The TD, who was a member of the Independent Alliance political group, said he had considered forming his own party, but instead decided to join forces with Mr Collins and Mr O’Donoghue.
“The time has come to build a new party that is the natural home for the majority of our people and politicians, and that will be a source of respectful dialogue, effective action and unwavering common sense for generations to come,” Fitzmaurice said in a statement. ” he said.
He said after “extensive negotiations” with Collins and O’Donoghue they decided “we can build something of substance together”.
“Too often independent parties and new parties split into narrow camps, when what is needed is unity, dialogue and sometimes compromise,” he said.
“I didn’t want to see that happen here.
“I believe that when it comes to elections, it is better for voters to have a clear and unambiguous choice. I believe that an independent Ireland can be that choice.”
He said an independent Ireland would be “a party of common sense solutions for this generation and future generations”.
Mr Fitzmaurice urged other TDs to “leave the comfortable ranks” of independent benches and establishment figures and “do the hard work of building something new”.
He campaigned under the banner of the Shane Ross Independence Alliance during the 2016 general election, but later left the political organization when he decided to join the Fine Gael government.
Independent Ireland leader Mr Collins said he was delighted Mr Fitzmaurice had “taken the plunge” to join his new party.
“Michael’s integrity and knowledge, combined with his experience in national politics, make him an excellent addition to the Independent Ireland Party,” he said.
Meanwhile, Mr O’Donoghue, general secretary of the Independent Ireland Party, said the party could not have looked for a “better partner”.
“His dedication, drive and determination are second only to his common sense approach to policy and priorities,” he said.
“We have a job ahead of us over the coming months and years, building the party not just for the next election but for future generations, but it is a job we are happy to take on and one we are sure will get done.” I am confident that we can do our part for the people of Ireland. ”
The newly formed party plans to publish a series of policy documents and an election manifesto, which it hopes will show voters and other politicians that there are alternatives to established parties.