The announcement Friday night fanned speculation that Crist would seek to regain his old job from Republican Gov. Rick Scott in 2014.
Crist sent out a tweet that said, "Proud and honored to join the Democratic Party in the home of President (at)Barack Obama!"
The tweet included a photo of a smiling Crist and his wife Carole as he held up a Florida voter registration application. The Tampa Bay Times reports that Crist signed the papers changing his affiliation from independent to Democrat at a Christmas reception at the White House. President Barack Obama greeted the news with a fist bump.
"I've had friends for years tell me, 'You know Charlie, you're a Democrat and you don't know it,'" Crist told the newspaper Friday night.
He cited the Republican Party's shift to the right on a range of issues, including immigration, education and the environment.
Messages left for Crist by The Associated Press weren't immediately returned Friday night.
Crist was elected Florida governor in 2006 while in the GOP. As he moved to run for the U.S. Senate in 2010, he faced a tough primary challenge from the right and bolted the GOP to run as an independent. He lost a three-way Senate contest in 2010 to Republican Marco Rubio.
Crist, 56, spoke at the Democratic National Convention in Charlotte, N.C., that nominated Obama for a second term and campaigned for his re-election.
Crist's decision to switch to a Democrat will increase speculation that he intends to challenge Scott, a former hospital chain CEO who has struggled with low favorability ratings since taking office. Crist has already criticized Scott for refusing to extend early voting despite pleas from U.S. Sen. Bill Nelson and other Democrats.
But it is unlikely that Crist would get a clear path to the Democratic nomination. Former State Sen. Nan Rich, D-Weston, has already jumped into the race and former Chief Financial Officer Alex Sink could run. Sink barely lost the 2010 governor's race to Scott. Some Democrats remain wary of Crist and even outgoing Florida Democratic Party chairman Rod Smith has joked that just because someone joins the congregation, "you don't make them the preacher."
Steve Schale, a Democratic political consultant who worked on Obama's Florida campaign, called a Crist a "viable Democrat."
"He earned his stripes when he supported the president," Schale said.
But Schale predicted that if Crist runs for governor, he would likely get a challenge from Sink and other Democrats and would have to endure a hard-fought primary.
Republicans in recent weeks have already ramped up their criticism of Crist and have pointed out that in the past he was critical of Obama and once described himself as a Republican in the mold of President Ronald Reagan and Crist's predecessor as governor, Jeb Bush.
"Charlie Crist's first official act as a Democrat was to tell a lie about why he is now pretending to be one," the Florida GOP said in a statement early Saturday. "The truth is that this self-professed, Ronald-Reagan Republican only abandoned his pro-life, pro-gun, conservative principles in 2010 after he realized that Republicans didn't want to send him to Washington D.C. as a senator, especially after he proved he couldn't do the job as governor."
Rick Wilson, a Florida-based Republican consultant, predicted that Crist would have to spend the next 14 months explaining how his switch to Democrat was something beyond just his own political ambitions.