Spitzer would be replaced by Lt. Gov. David Paterson, who will become New York's first black governor.
The scandal erupted Monday when allegations surfaced that Spitzer, a 48-year-old married man with three teenage daughters, spent thousands of dollars on a call girl named Kristen at a swanky Washington hotel on the night before Valentine's Day.
"I have acted in a way that violates my obligations to my family and violates my -- or any -- sense of right and wrong," the governor said at a news conference with his wife, Silda, at his side. "I apologize to the public, whom I promised better."
Calls for his resignation came immediately. Republicans began talking impeachment if he didn't step aside. Meanwhile, Spitzer stayed holed up in his Manhattan apartment, where he was reportedly weighing his options, including waiting to use resignation as a bargaining chip with federal prosecutors to avoid indictment.
The case involving Spitzer started when banks noticed frequent cash transfers from several accounts and filed suspicious activity reports with the Internal Revenue Service, a law enforcement official told The Associated Press. The accounts were traced back to Spitzer, leading public corruption investigators to open an inquiry.
A law enforcement official said Tuesday that Spitzer had spent tens of thousands of dollars with the call-girl service Emperors Club VIP. Another official said the amount could be as high as $80,000.
Still another law enforcement official said investigators found that during the tryst with Kristen, Spitzer used two rooms at Washington's Mayflower Hotel -- one for himself, the other for the prostitute. Sometime around 10 p.m., Spitzer sneaked away from his security detail and made his way to her room, the official said.
According to an affidavit, a federal judge approved wiretaps on the escort service's telephone in January and February. FBI agents in Washington had the Mayflower under surveillance when Spitzer was in town, a senior law enforcement official said.
The officials spoke to the AP on condition of anonymity because of the sensitivity of the case.
Spitzer, a first-term Democrat, built his political reputation on rooting out government corruption, and made a name for himself as attorney general as crusader against shady practices and overly generous compensation. He also cracked down on prostitution.
He was known as the "Sheriff of Wall Street." Time magazine named him "Crusader of the Year," and the tabloids proclaimed him "Eliot Ness." The square-jawed graduate of Princeton University and Harvard Law was sometimes mentioned as a potential candidate for president.
He rode into the governor's office with a historic margin of victory on Jan. 1, 2007, vowing to stamp out corruption in New York government in the same way that he took on Wall Street executives with a vengeance while state attorney general.
His term as governor has been fraught with problems, including an unpopular plan to grant driver's licenses to illegal immigrants and a plot by his aides to smear his main Republican nemesis. The prostitution scandal, some said, was too much to overcome.
Freshman Rep. Kirsten Gillibrand became the first Democratic member of New York's congressional delegation to mention resignation on Tuesday. "This is very grave and sad news," she said. "If these serious allegations are true, the governor will have no choice but to resign."