The former Florida governor, 61, said he made the decision over the Thanksgiving holiday in consultation with his family.
"As a result of these conversations and thoughtful consideration of the kind of strong leadership I think America needs, I have decided to actively explore the possibility of running for President of the United States," he said in a message posted on Facebook today.
“In January, I also plan to establish a Leadership PAC that will help me facilitate conversations with citizens across America to discuss the most critical challenges facing our exceptional nation. The PAC’s purpose will be to support leaders, ideas and policies that will expand opportunity and prosperity for all Americans,” Bush said in the message, which he also tweeted. "In the coming months, I hope to visit with many of you and have a conversation about restoring the promise of America.”
His timing is to send the message to donors, GOP activists and other 2016 prospects that he is dead-serious about running and they should take that into account, a top Republican close to Bush told ABC News.
In short, hold off and don't sign on with anyone else.
This could be bad news for Florida Sen. Marco Rubio, who was already planning a run. But Rubio will not base his decision on Bush's exploratory committee, an aide told ABC News in the hours after today's announcement.
"Marco has a lot of respect for Governor Bush, and believes he would be a formidable candidate," Rubio spokesman, Alex Conant, told ABC News. "However, Marco's decision on whether to run for President or re-election will be based on where he can best achieve his agenda to restore the American Dream -- not on who else might be running."
Asked about Bush today, another potential 2016 hopeful, Sen. Rand Paul, R-Ky., told reporters "the more the merrier."
"I think we need to have a big tent and we can use moderates, conservatives, libertarians," Paul said, declining to comment on whether Bush's announcement affects his own time-frame for a presidential bid.
In a mid-October ABC News-Washington Post poll, assuming Mitt Romney is not a candidate, Bush had 13 percent support among Republicans and Republican-leaning independents when it came to GOP presidential primary preference. That puts him essentially alongside Mike Huckabee (12 percent), Rand Paul (12 percent), Paul Ryan (9 percent), Marco Rubio (9 percent) and Chris Christie (8 percent).
Bush does best among party regulars rather than independents, which can help in most primaries. But his 13 percent support means that 87 percent of leaned Republicans prefer someone else (or none of the above). And he’s got some baggage; in another ABC-Post poll in late October, 52 percent of registered voters said they thought Bush would not make a good president. He has also trailed Hillary Clinton in head-to-head matchups in ABC-Post polling and others.
Bush, who served as governor of Florida from 1999-2007 and is the son of former President George H.W. Bush and the brother of former President George W. Bush, delivered the commencement address at the University of South Carolina Monday, but offered no hints about today’s announcement. Instead, he gave graduates three pieces of advice.
“Dream big, don’t be afraid of change and find joy everywhere you can,” Bush told the attendees.
ABC News’ Michael Falcone, Jeff Zeleny, Gary Langer, Rick Klein, Shushannah Walshe and Arlette Saenz contributed reporting.