New York needs more millionaires — or risks falling behind its rivals and losing billions in tax revenue, a fiscal watchdog warned in a new report that’s already roiling the mayor’s race.

The nonpartisan Citizens Budget Commission said Thursday that while New York state nearly doubled its number of millionaire earners from 2010 to 2022, other states blew past it.

California, Florida and Texas all added millionaires at a faster clip — more than tripling their totals.

That gap translates into real money. If New York had kept pace, the watchdog said, the state and city together would have collected an extra $13 billion in taxes in 2022 alone.

That slide has cost taxpayers dearly. If New York had simply held its ground, the state would have raked in an extra $10.7 billion in 2022 alone — and the city another $2.5 billion.

In 2021, the missed windfall would have topped $15.3 billion, thanks to soaring Wall Street gains.

Instead, New York has fallen from second to fourth in the nation for millionaire households, leapfrogged by Florida and Texas. California remains No. 1, but even high-tax California is adding rich residents at a far faster clip.

Florida quadrupled its millionaire ranks between 2010 and 2022, while Texas and California more than tripled theirs. Meanwhile, New York’s millionaire population merely doubled.

“People choose where to live based on a whole mix of factors,” Andrew Rein, the commission’s president, told The Post.

“Taxes matter, but so do quality of life, affordability, public services, and economic opportunities.”

Millionaires remain the linchpin of New York’s finances. They make up just 1% of taxpayers but generate a staggering 40% of city personal income tax revenue and 44% of the state’s.

In 2022, they contributed $34 billion to state and city coffers, including $28 billion from residents alone.

But their numbers are growing faster elsewhere. In 2010, New York City had as many millionaires as all of Florida. By 2022, Florida had 56% more.

Florida is increasingly attracting clients seeking a combination of financial and lifestyle benefits, according to Jay Batra, a New York– and Florida-based real estate broker.

“The state’s tax advantages, pro-business climate, and vibrant economy are major draws, while its stunning beaches, year-round sunshine, and relaxed lifestyle offer a compelling quality-of-life upgrade,” Batra told The Post.

He added that many of his New York-based clients who relocate to Florida are choosing to retain their Manhattan residences, “at least for now.”

“Over time, most expect to transition to a smaller pied-à-terre in the city as work and lifestyle needs change.”

New Yorkers aren’t just focusing on decamping to warmer climates. Some are considering relocating closer to home.

Libby McKinney Tritschler, a Fairfield County luxury properties specialist, said she’s spotting a “significant trend” among her New York clients who own secondary homes in Connecticut.

Many are now upgrading their weekend or summer residences in anticipation of a more permanent move — with some decisions hinging on the outcome of the upcoming New York mayoral election.

Tritschler told The Post that she estimated she has seen at least 25 high-net-worth individuals make the move in recent years.

Roughly half are leaving New York full-time, she added, while the other half are downsizing their city footprint by selling larger apartments, purchasing pied-à-terres or investing in larger Connecticut homes to host their growing families.

The trend, she said, is reshaping the market in trophy enclaves like Southport.

“This is creating incredible demand in the trophy enclaves of CT, which already had limited inventory,” Tritschler said, pointing to a listing she has at 260 Willow Street in Fairfield that’s been drawing steady interest from New York families.

Momentum is building, she added, because “news spreads like wildfire.” One person buys in Southport, she said, and quickly convinces their friends to do the same.

“There is such limited inventory that it creates a sense of urgency when something does come to market.”

“We see fewer millionaires moving here and more moving out,” Rein told The Post.

“New York just isn’t growing its share of top earners the way competitor states like California, Texas, and Florida are.”

The report warns New York’s “value proposition” has faltered: sky-high taxes, battered quality of life, crime worries, hybrid work and soaring housing costs have dulled the city’s appeal.

“Simply put, New York’s value proposition has not been attractive enough to keep up with millionaire growth nationwide,” the report concluded.

Making matters worse, Gotham now punishes its highest earners with the steepest tax rates in America. A New Yorker making $25 million a year pays 14.776% in state and city income tax, compared to 13.3% in California.

Even those earning $2.2 million pay more in New York than they would on the West Coast.

The findings land just nine weeks before voters in the five boroughs head to the voting stations to case their ballots in an election that has been consumed by concerns about affordability and inequality.

The Citizens Budget Commission’s push to hold onto wealthy New Yorkers echoes the arguments of Mayor Eric Adams and former Gov. Andrew Cuomo, who both say the city can’t afford to drive out its top earners.

“I don’t want to say to billionaires, ‘We don’t want you here,’” Adams said on a recent podcast.

“I know why we need them here: The money we make just on stock transfer taxes and bonuses, that actually impacts our budget.”

Cuomo blasted rival Zohran Mamdani’s plan to slap a new 2% millionaire tax as “class warfare.”

Mamdani, the Democratic nominee, wants to use the money for free child care and free bus rides — and has gone so far as to declare that “billionaires should not exist.”

Rein told The Post that raising taxes could accelerate — and exacerbate — the trend of millionaires preferring other places to live.

“New York State and its localities have the highest taxes and the second-highest spending in the nation,” he said.

“We should be able to hold the line on taxes. Raising them could risk accelerating the trend of our shrinking share.”