Mr Cohen’s lawyer, Lanny Davis, took to Twitter to announce her client had decided to rejoin the US’s left-wing party.
According to the attorney, Mr Cohen was doing so to mark “another step in his journey distancing himself from the values of the current Administration.”
She also added he was “putting family and country first”.
Mr Cohen later retweeted his lawyer’s tweet.
Prior to June, the President’s longtime former personal attorney served as the Republican National Committee’s (RNC) deputy finance chairman.
Mr Cohen was forced to resign after he began to be investigated by federal prosecutors from bank fraud, tax fraud and for violating campaign finance law.
During the 2016 presidential campaign, Mr Cohen was a registered Democrat.
The attorney changed his party affiliation after he took the leadership role with the RNC once Trump was elected.
Last August, Mr Cohen pleaded guilty to the charges of bank and tax fraud as well as for violations of campaign finance laws where he made payments to buy the silence of two women who allegedly had affairs with the then-candidate Trump in 2006.
He claims that he did this on Trump’s orders.
Mr Cohen’s plea deal did not include an agreement to cooperate with the Special Council investigating Russian interference in the 2016 election.
However, the President’s former attorney agreed to voluntarily sit down with Robert Mueller and his time to answer their questions.
The prosecutors from the US Attorney’s Office for the Southern District of New York were also in attendance for the interviews.
Mr Cohen has also cooperated with a separate investigation into the Trump Foundation by New York state investigators.
Trump has consistently downplayed his connection to Mr Cohen after his former attorney pleaded guilty to New York’s Southern District.
After Mr Cohen accepted the plea deal, the President argued that “flipping” to operate wit prosecutors “almost ought to be illegal.”