WASHINGTON — Scott Pruitt, the administrator of the Environmental Protection Agency and architect of President Trump’s aggressive effort to rewrite the government’s rule book on environmental regulations, resigned on Thursday in the face of numerous ethics investigations that doomed his tenure.

Despite Mr. Pruitt’s efforts to nurture a close relationship with the president, Mr. Trump himself announced the resignation in a tweet sent from Air Force One. He thanked Mr. Pruitt for an “outstanding job” and said the agency’s deputy, Andrew Wheeler, a former coal lobbyist, would take over as the acting administrator on Monday.

Mr. Pruitt in his resignation letter cited “unrelenting attacks on me personally” as one of the reasons for his departure. Mr. Pruitt had been hailed by conservatives for his zealous deregulation, but he could not overcome a spate of questions about his alleged spending abuses, first-class travel and cozy relationships with lobbyists.

Mr. Pruitt also came under fire for enlisting aides to obtain special favors for him and his family, such as reaching out to the chief executive of Chick-fil-A, Dan T. Cathy, with the intent of helping Mr. Pruitt’s wife, Marlyn, open a franchise of the restaurant.

The resignation appeared to happen quickly.

On Wednesday, Mr. Pruitt attended two Fourth of July parties, one at the White House and another at the Interior Department. One attendee who spent time with him said he spent the night mingling, shaking hands, watching the fireworks and showing no indication that he planned to step down. His chief of staff, Ryan Jackson, also gave no hint of what was ahead.

An individual close to Mr. Pruitt said the president acted after he found one particular story in recent days embarrassing: a report that Mr. Pruitt had asked Mr. Trump to fire Jeff Sessions, the attorney general, so that Mr. Pruitt could run the Justice Department.

The idea had been discussed privately for months by the president, who occasionally asked advisers if it was a good idea, according to two people familiar with the discussions. But seeing those deliberations being aired publicly, amid a string of other damaging reports, focused Mr. Trump’s attention, a person close to the president said. Fresh allegations that Mr. Pruitt had retroactively altered his public schedule, potentially committing a federal crime, had also escalated concerns about him at the White House, according to a White House aide. On Thursday afternoon, around 1:30, Mr. Trump’s chief of staff, John F. Kelly, reached out to Mr. Pruitt to tell him the time had come.

Mr. Pruitt, a former Oklahoma attorney general who built his career on lawsuits against the agency he would eventually lead, remained a favorite of Mr. Trump’s for the majority of his tenure at the E.P.A. He began the largest regulatory rollback in the agency’s history, undoing, delaying or blocking several Obama-era environmental rules. Among them was a suite of historic regulations aimed at mitigating global warming pollution from the United States’ vehicles and power plants.

[Mr. Wheeler shares Mr. Pruitt’s zeal to dismantle climate change regulations. Read more here.]

Mr. Pruitt also played a lead role in urging Mr. Trump to follow through on his campaign pledge to withdraw the United States from the 2015 Paris climate agreement, despite warnings from some of the president’s other senior advisers that the move could damage the United States’ credibility in foreign policy. Under the landmark accord, nearly every country had committed to reducing emissions of planet-warming fossil fuel pollution.

In 2017, Mr. Pruitt made headlines for questioning the established science of human-caused climate change, contradicting decades of research by scientific institutions, including his own agency. Although Mr. Pruitt was harshly criticized for the remarks, they did not affect his good standing with a president who has also mocked climate science.

Mr. Trump has repeatedly told associates that Mr. Pruitt has done what he has wanted in terms of cutting regulations, so he has been reluctant to let him go. Mr. Pruitt, for his part, had made himself available to the president as a confidant.

He ingratiated himself in part by offering himself as a sounding board on topics ranging from the North American Free Trade Agreement to to the Russia investigation, and he would join in as the president criticized the attorney general, Mr. Sessions. He often lunched at the White House mess in hopes of running into the president.

But White House advisers, including Mr. Kelly, the president’s chief of staff, for months have implored Mr. Trump to get rid of Mr. Pruitt. Ultimately, the president grew disillusioned with Mr. Pruitt after the accusations of impropriety and ethical missteps overshadowed Mr. Pruitt’s policy achievements.

In recent days, people who have spoken with Mr. Trump said he sounded exasperated with his E.P.A. administrator’s negative headlines. “It’s one thing after another with this guy,” one person close to Mr. Trump quoted the president as saying.

Mr. Pruitt is the subject of at least 13 federal investigations, and a government watchdog agency concluded that he had broken the law with his purchase of a $43,000 secure telephone booth. He was also under investigation for his 2017 lease of a bedroom in a condominium linked to a Canadian energy company’s powerful Washington lobbying firm, and for accusations that he demoted or sidelined E.P.A. employees who questioned his actions.

The former E.P.A. administrator had come under criticism for lavish expenditures on foreign travel, including a trip arranged for him by a lobbyist to Morocco, a country where the E.P.A. has no policy agenda. His domestic travel also came under fire after a former staff member told congressional investigators that his boss often sought to travel to Oklahoma, where Mr. Pruitt owns a home, directing his employees to “find me something to do” there so he could justify charging taxpayers for the expense.

A New York Times report earlier this year detailed Mr. Pruitt’s lavish spending and questionable practices in his home state. O n Thursday The Times reported on new questions about whether aides to Mr. Pruitt had deleted sensitive information about his meetings from his public schedule, potentially in violation of the law.

While Democrats have criticized Mr. Pruitt since his nomination, in recent months even conservative Republicans had taken the unusual step of criticizing and questioning his ethics. Representative Trey Gowdy, Republican of South Carolina and the chairman of the House Oversight Committee, has started an investigation into Mr. Pruitt’s actions at the E.P.A., the first such Republican-led inquiry into a Trump administration cabinet member.

Joni Ernst and Charles E. Grassley, two Republican senators from Iowa, a farm state with a solid bank of Trump voters, have both publicly criticized Mr. Pruitt. Last month, the conservative National Review, which once championed his appointment, called on Mr. Pruitt to resign. On Tuesday, Laura Ingraham, the conservative talk radio host, tweeted, “Pruitt is the swamp. Drain it.”

On May 2, Mr. Gowdy’s staff began conducting transcribed, behind-closed-doors interviews with Mr. Pruitt’s closest aides. Partial transcripts from one of those interviews revealed that Mr. Pruitt used one of his top aides last year essentially as a personal assistant, having her help him search for an apartment as well as try to procure a used mattress from the Trump International Hotel.