ALBANY, N.Y. — New York State officials have reached a deal with the Buffalo Bills to help the team build a $1.4 billion stadium adjacent to its current home using $850 million in public funds, the largest taxpayer contribution ever for a pro football facility.

Under the deal, the state would finance $600 million of the construction costs, while Erie County, where the stadium will be built, would cover $250 million. The remainder would be financed through a $200 million loan from the N.F.L. that was approved on Monday, plus $350 million from the team owners.

The public dollars, which still need to be approved by lawmakers, would cover about 60 percent of the projected construction costs, a percentage that is slightly lower than in recent stadium deals in similarly small markets in the N.F.L. But the overall subsidy is the largest since Clark County in Nevada issued $750 million in bonds to help pay for the construction of a new arena before the Raiders moved to Las Vegas in 2020.

The announcement came after months of secretive back-room negotiations among Gov. Kathy Hochul, county officials and the team owners, who indicated last year that they would not renew the lease of the current arena, Highmark Stadium, which is nearly 50 years old, one of the oldest professional football stadiums in the country.

The team pursued public funding for a new stadium instead, with state officials agreeing that building a new arena made more financial sense than undertaking renovations that could have been almost as expensive.

Striking a deal to ensure that the Bills remained in Western New York was a top priority for Ms. Hochul, a Buffalo native and avid Bills fan who took office in August and is running for a full term as governor this year.

In a statement on Monday, Ms. Hochul said officials had secured a 30-year commitment from the team to play in the new stadium, which she said would create 10,000 union construction jobs. The state would own the stadium and lease it to the Bills.

“I went into these negotiations trying to answer three questions,” Ms. Hochul said. “How long can we keep the Bills in Buffalo, how can we make sure this project benefits the hard-working men and women of Western New York and how can we get the best deal for taxpayers?”

Julie Wood, a spokeswoman for the governor, said that the state portion would be funded through “a mix of new and existing capital appropriations” that Ms. Hochul would ask lawmakers for in this year’s budget.

Ms. Hochul, a Democrat, unveiled the agreement just four days before the state budget is due in Albany, giving state lawmakers a relatively small window to scrutinize the details of the state portion of the financing that they must ultimately decide whether to approve.

The subsidy is bound to make the already fraught state budget negotiations even more contentious, with some left-wing Democrats from the downstate region already decrying the public funding as corporate welfare.

Indeed, the negotiations over a new stadium rekindled a bitter debate about whether government should be in the business of subsidizing arenas for professional sports teams; economic research has found that sports stadiums have rarely had a substantial impact, if any, on overall economic growth.

Public assistance, in the form of tax breaks and free land, has been used to finance the construction of arenas for New York sports teams, but many of the teams, from the Yankees to the Mets, have financed most of the costs themselves. The Giants and the Jets, which play in New Jersey, paid for nearly all of their stadium, which opened in 2010.

The negotiations also sparked skepticism about whether the Bills would have ultimately abandoned New York without a large government subsidy, though the owners never publicly threatened to uproot the team.

The Bills are owned by Pegula Sports and Entertainment, a company helmed by Terry and Kim Pegula, a wealthy couple based in Western New York; Mr. Pegula, who made his fortune through fracking, has a net worth of $5.8 billion, according to Forbes.

The new open-air stadium, which would be built across the street from the Bills’ current home in Orchard Park, a Buffalo suburb, would hold just over 60,000 fans, about 10,000 fewer than the current venue. But it would have a larger footprint overall and include about 60 box suites, a more lucrative source of revenue for teams.

A partial roof would cover about 80 percent of all seats, an attempt to protect fans from the region’s frigid wind and snowfalls during winter games. The new stadium could open as early as 2026 if construction begins within a year, according to team officials.

The N.F.L., which supported the idea of a new stadium, approved the $200 million loan for the construction during a spring meeting in Florida on Monday morning, shortly before Ms. Hochul’s announcement.

Local legislators in Erie County must also approve the county’s part of the financing.

Ken Belson contributed reporting.