Prosecutors say that Mr. Yoon committed insurrection when he banned all political activities and ordered military commanders to break the National Assembly’s doors down “with axes” or “by shooting, if necessary” and “drag out” lawmakers.

Mr. Yoon has repeatedly rejected the claims of orchestrating an insurrection. He said that he declared martial law to protect the nation from “anti-state forces” who infiltrated the government.

If the court finds Mr. Yoon guilty, he could face a long prison term. But many of South Korea’s former imprisoned presidents, including Park Geun-hye, who was convicted of bribery, have ended up being released early. Ms. Park was pardoned in 2021 by then-president Moon Jae-in, less than five years into her 20-year prison term.

The criminal case will not be directly affected by the Constitutional Court ruling that upheld his impeachment.

Mr. Yoon’s time in office was plagued with problems and scandals. After his narrow electoral win in 2022, he soon became a deeply unpopular and divisive leader.

As president Mr. Yoon used lawsuits, state regulators and criminal investigations to clamp down on speech that he called disinformation, efforts that were largely aimed at news organizations. The police and prosecutors repeatedly raided the homes and newsrooms of journalists whom his office accused of spreading “fake news.”

Mr. Yoon was also accused of using his power to advance his own interests. He was accused of pressuring the Defense Ministry to whitewash an investigation into the death of a South Korean marine in 2023, and vetoed a bill pushed through Parliament by the opposition calling for a special prosecutor to investigate the claim.

Mr. Yoon’s wife, Kim Keon Hee, was also at the center of some of his troubles. In late 2023, spy cam footage emerged showing Ms. Kim accepting a $2,200 Dior pouch as a gift. The episode roiled his political party and became a significant issue ahead of parliamentary elections.

Relations with North Korea sank to longtime lows after Mr. Yoon took office. For decades, the two Koreas — which never signed a peace treaty after the 1950-53 Korean War ended in a truce — have swung between conciliatory tones and saber rattling. North Korea’s leader, Kim Jong-un, has been unpredictable and bellicose, developing nuclear weapons and supplying Russia with munitions and troops for its war against Ukraine.

Mr. Yoon adopted a confrontational approach and called for spreading the idea of freedom to the North to penetrate the information blackout there. He also expanded joint military drills with the United States and Japan.

North Korea, under Mr. Kim, has veered toward a more hawkish stance, shutting off all dialogue with Seoul and Washington, doubling down on testing nuclear-capable missiles, and vowing to treat South Korea not as a partner for reunification but as an enemy that the North must annex should war break out.

Mr. Yoon leaves behind a government without an elected leader and facing a presidential poll within 60 days.

“It has been a great honor for me to work for the country,” Yoon Suk Yeol said in a statement released through his lawyers after he was removed from office. “I thank those who supported and cheered me despite my shortcomings. I am regretful and sorry that I could not live up to your expectations,” he said, without directly commenting on the Constitutional Court’s decision.

Mr. Yoon and his wife will need to vacate the official residence where he once holed up to resist arrest, protected by barricades and scores of security guards.

The couple is expected to move back into their private home in Seoul, where they had lived before Mr. Yoon’s election. But they will continue to have the protection of the presidential security services — one of the few benefits allowed for a former leader ousted through impeachment.

As the acting chief justice began reading out the decision, filled with technical legal jargon, the crowds grew quieter, straining to hear from outdoor loudspeakers the outcome that would determine Mr. Yoon’s fate — and that of the country. Some people opposed to Mr. Yoon cheered and applauded intermittently as the reading went on. Protesters on both sides clasped their hands together in prayer. Many held up their phones to record the moment.

And then, the crowd erupted.

Those supporting Mr. Yoon’s impeachment reacted with hugs, screams and fists pumped in the air to the unanimous decision to remove him from office. At a rally of supporters of Mr. Yoon near his residence, there was loud booing. Some people folded over in dismay, and others loudly cursed.

“It’s been so hard, but now my heart is relieved,” said Kim Ji-seon, 55, who brought her daughter to witness the verdict in the hopes that Mr. Yoon’s impeachment would be upheld. “I hope they can create a country where people can be happy.”

Soon after the decision was announced, only a handful of people, some wearing “Make Korea great again” and “stop the steal” hats, were left at a pro-Yoon demonstration that was scheduled to last all day outside the presidential office. Construction workers were taking down scaffolding, and dozens of chairs had been stacked and pushed to the side.

Jang Jaeeuk, 21, said he had stayed out on the street near the court overnight along with other students from his university, getting only three hours of sleep, because it was such an important day for his country.

“I’m happy that the hardships of the past four months haven’t gone to waste,” he said after crying and hugging his fellow students upon hearing the verdict. “Now I feel that we can change the world and, in that sense, I’m hopeful for the future.”

Supporters of Mr. Yoon near the court, who were predominantly men, said that they were deeply disappointed that the president was removed and went home.

Yoon Seo-jun, 18, sitting on the edge of a sidewalk, said he was distrustful of the system that led to the impeachment and dismayed about the future of the country.

“South Korea’s democracy is dead,” he said, adding that he hoped the National Assembly, which had impeached Mr. Yoon, would be dissolved.

After the decision, some South Koreans expressed apprehension about what might come next and the possibility of further political upheaval.

Lee Yongseok, 27, who watched the court’s televised decision at a screen at Seoul’s main railway station, said that although he supported the verdict, he knew that others would be skeptical of the ruling and of the judges who made it.

“I feel like something big is going to come to my country,” he said.

Even as supporters and opponents of Mr. Yoon tried to grasp what comes next for their country, government officials were taking steps to move on.

In a gesture that underlined the finality of the court’s ruling, the officials removed the presidential emblem in front of the building where Mr. Yoon used to work as president. The emblem bears the image of a mythical bird, the phoenix.

Choe Sang-Hun, Chang W. Lee, Jun Michael Park and Victoria Kim contributed reporting.

At 11 a.m. local time, the court’s acting chief justice, Moon Hyungbae, began reading the ruling, which came after months of deliberation. Some 22 minutes later, he announced that all eight of the court’s justices had voted to remove Mr. Yoon from office. The now ex-president had “betrayed the trust of the people” and “severely violated the law,” Mr. Moon said.

“It has been a great honor for me to work for the country,” Mr. Yoon said in a statement released through his lawyers. “I and regretful and sorry that I could not live up to your expectations.”

The court ruling is effective immediately and cannot be appealed.

Why was Mr. Yoon impeached?

Late on Dec. 3, Mr. Yoon stunned the country when he declared martial law, but it lasted only six hours because lawmakers quickly voted to overturn it.

Shortly after he made the announcement on live television, troops stormed the National Assembly building under orders to “break down the door and drag out” lawmakers gathering inside to vote down the martial law decree and arrest key figures, including the speaker of the floor. Crowds of citizens confronted the troops, and some lawmakers scaled fences to get into the assembly and vote.

On the same night, Mr. Yoon also sent troops to the National Election Commission who attempted to search and confiscate its database. This violated the agency’s constitutionally guaranteed independence, the court ruled.

Delivering the ruling, Justice Moon said Mr. Yoon had abused his duties as the nation’s commander in chief. The martial law decree did not meet the constitutional requirement for a national crisis which Mr. Yoon repeatedly said was one of the reasons he had made the extraordinary declaration.

The judges said that Mr. Yoon rushed his idea for imposing martial law through a hurriedly convened Cabinet meeting on the night of Dec. 3. without proper deliberation, and that he failed to notify the National Assembly, as was required by law.

Only six of the eight members on the bench needed to vote in favor to uphold the impeachment, but the decision was unanimous.

What will happen next?

Acting president Han Duck-soo will continue in his role until the nation elects a new president through a snap election. This must take place within 60 days from Friday, meaning early June at the latest. The National Election Commission is expected to announce the date in the coming days.

Political parties will select their candidates through primary elections.

Lee Jae-myung is the expected front-runner to lead the opposition Democratic Party. He has been Mr. Yoon’s political rival since the last presidential elections in 2022 when he lost to the former president by less than 1 percent of the votes. Mr. Lee has been at the helm of the political push to remove the now-impeached president.

It’s less clear who will come out in front of the race to lead the ruling People Power Party. Its former leader, Han Dong-hoon, resigned in December after disagreement with both Mr. Yoon and his party over the martial law issue.

Oh Se-hoon, the mayor of Seoul, and Hong Joon-pyo, the mayor of Daegu, are others who have been suggested as potential candidates.

The first protests were set to start at 10 a.m. local time, ahead of a possible decision by the court that could come as early as 11 a.m. Mr. Yoon was not expected to attend the reading of the decision, which will determine whether he will be formally removed from office or returned to power.

Millions of South Koreans have protested, mostly peacefully, since Mr. Yoon briefly declared martial law on Dec. 3, plunging the nation into political upheaval and prompting lawmakers to impeach him. Mr. Yoon was detained in January on insurrection charges but released unexpectedly last month after a Seoul court said his detention was procedurally flawed.

Ahead of the Constitutional Court’s decision, which cannot be appealed, there have been growing worries that Mr. Yoon’s supporters will clash with those demanding he be removed from office. After his arrest in January, some of Mr. Yoon’s supporters overran a local court, breaking windows and threatening the judge hearing his case.

The police have created a fortresslike environment outside the Constitutional Court, placing 15-foot-tall metal barriers on either side of the main avenue approaching the complex in an effort to keep the two camps from confronting each other. Between the added security gates, police have parked dozens of buses and put up smaller metal fences to deter people from occupying the area.

The police response is not without precedent. In 2017, thousands of people tried to cross the police barricade near the court after it ruled that the president, Park Geun-hye, should be removed from office. At least four people died during the protests.

On the eve of the Constitutional Court’s ruling this time, South Korea’s interim leader, Han Duck-soo, called on the nation to respect the decision “with calm.”

“The government will not tolerate any illegal or violent acts,” he said on Wednesday, urging politicians not to incite violence. “This is a time to put the stability and fate of our community ahead of political interests.”