Bloomberg gives millions each year to benefit medical research, arts and education, but he typically doesn't announce he is doing so.
On Tuesday, he made an exception and released a statement about the anti-smoking campaign, which he described as "a social investment."
"It's based on my experience that with better data, more focused public health interventions and heightened political advocacy, governments will enact policies that have huge health, social and economic benefits," Bloomberg said.
The $125 million will go to existing organizations over two years to implement anti-smoking measures. The groups that will receive the funds have not been selected.
His effort will include cash for programs that help smokers quit and educate children to prevent them from starting; funds to push for smoking bans and higher tobacco taxes in other cities, states and countries; and money for a system to track global tobacco use and the effectiveness of anti-smoking efforts.
"Unless we take urgent action, this century a billion people will die from smoking," Bloomberg said. "It is one of the world's biggest killers, and it has sadly been overlooked by the philanthropic community."
The campaign mirrors what the Republican mayor already started in New York, where he banned smoking in bars and restaurants and has the health department running an aggressive program focused on helping smokers quit. Nearly 1.2 million New Yorkers smoke, and health officials have given out thousands of free nicotine patches.
According to the World Health Organization, an estimated 1.3 billion people worldwide are smokers. In 2003, the WHO adopted an anti-tobacco treaty that requires participants to restrict tobacco advertising, put tougher health warnings on cigarettes, enact tax hikes and install smoking bans.
Now that Bloomberg is in his last term and still enjoying sky-high approval numbers, every word he utters and move he makes outside city limits stirs speculation about his future plans. He has said repeatedly he will not run for president and plans to concentrate on full time philanthropy when he leaves City Hall in 2009.
Bloomberg built his wealth from the financial information company he founded, Bloomberg L.P., but stepped down as CEO when he first ran for mayor. His fortune is estimated at $5.1 billion, making him the 40th richest American and the 112th wealthiest person in the world, according to Forbes magazine.