"Today, on behalf of an unprecedented collection of US cities, states, businesses and other organizations, I am communicating to the United Nations and the global community that American society remains committed to achieving the emission reductions we pledged to make in Paris in 2015," Bloomberg said in a statement.
"I am confident the broad array of leaders and organizations that have signed today's declaration, and many others that will join in the days to come, will work together to reduce US carbon emissions by 26 percent by 2025, just as we had pledged in Paris."
Big names like Apple, Amazon, Google, Facebook, Microsoft, Levi Strauss and Tiffany are in the group, called "We Are Still In," along with more than 100 mayors and governors and a range of colleges and universities.
A full list of the signatories is available at wearestillin.org.
The group called Trump's decision to pull the United States out of the climate accord "a grave mistake that endangers the American public and hurts America's economic security and diplomatic reputation."
"In the absence of leadership from Washington, states, cities, colleges and universities and businesses representing a sizeable percentage of the US economy will pursue ambitious climate goals," it said in a statement.
At least 211 mayors have adopted the Paris goals for their own cities since Trump's announcement on Thursday, and at least 17 governors have released individual statements saying they stand by the Paris deal, agreed in late 2015 by every country in the world except Syria and Nicaragua.
Bloomberg, a billionaire climate advocate, also met with French President Emmanuel Macron on Friday to tell him the American people intend to uphold the pact.
Bloomberg has also pledged to muster $15 million for the United Nations' climate body, substituting for US funding likely to be axed by Trump.
Bloomberg, 75, was mayor of New York from 2002 to 2013.
Forbes magazine estimates him to be the world's eighth-richest person.
Although Bloomberg's initiative is not the only pushback against Trump's move, it is the most prominent, said Steve Cohen, executive director of The Earth Institute at Columbia University.
"You are seeing a massive response to Trump's decision," he said, noting the United States has already begun to turn away from coal in favor of natural gas and renewable forms of energy.
"I think the US will exceed the Paris agreement targets," he added.
Darren Rosenblum, law professor at Pace University in New York, says the business community is coming together to address the most direct threat posed by a US pullout -- loss of competitiveness as the rest of the world barrels toward a greener economy.
"I do think that in the end, the effect is a positive one," he said.
"It starts to put in place the mechanisms for US companies and local governments to follow international norms."
