A court in Saudi Arabia has sentenced to death the brother of an exiled dissident, convicting him of disloyalty to the kingdom’s rulers in a case built around anonymous social media accounts where he shared criticism of the government.

The defendant, Mohammed bin Nasser al-Ghamdi, a 54-year-old retired teacher, had almost no public profile before he was arrested last year and accused of treason. One of the main social media accounts cited in his court case, on the platform X — formerly known as Twitter — has eight followers.

The sentence, which was handed down in July, was also based on a confession attributed to Mr. al-Ghamdi after his arrest, in which he said he viewed the king and crown prince as “tyrants” and “agents of the West” who were fighting against Islam, according to court documents reviewed by The New York Times.

One possible explanation for his prosecution was offered by his elder brother, Saeed bin Nasser al-Ghamdi, a conservative Muslim cleric and vocal dissident who lives in exile in Britain. He said the authorities seemed to be using his younger brother to punish him.

“The posts that my brother wrote, no one knew about, and they didn’t spread — no one saw them even,” Saeed al-Ghamdi told The Times on Friday. “It appears to me they wanted to spite or harm or try to disturb me with this case.”

The case is part of a crackdown on dissent that has deepened under Crown Prince Mohammed bin Salman, 38, the kingdom’s de facto ruler.

Over the past eight years, the prince has rendered the once-ultraconservative country nearly unrecognizable, overseeing a plan to diversify its oil-dependent economy and ending a slew of religious and social restrictions that many Saudis found suffocating. At the same time, the modest space for political discourse has contracted.

Since 2017, the Saudi authorities have arrested hundreds of critics across the political spectrum, including religious clerics, Snapchat influencers, billionaires and several of the prince’s own cousins. The 2018 killing of Jamal Khashoggi, the Washington Post columnist, by Saudi agents in Istanbul, which drew international outrage, was the most brazen example of the push. A former Saudi government insider, Mr. Khashoggi had fled the country and become an outspoken critic.

In recent years, harsher punishments than ever are being meted out to citizens who criticize their government, even as the accused have become less prominent.

After being detained, the younger Mr. al-Ghamdi confessed to holding religious and political beliefs that prosecutors portrayed as grave violations of the kingdom’s broadly worded counterterrorism law, according to the court documents. In the confession attributed to him after his arrest, he acknowledged that he was behind the anonymous social media accounts in question.

His lawyer denied the charges and said that his client “loved and was loyal to this nation,” according to the court documents. The lawyer argued that his client had neurological and psychiatric conditions that should void any statements attributed to him.

Despite that, a panel of judges sentenced Mr. al-Ghamdi to death, according to a copy of the ruling. The verdict is open to appeal.

The Saudi government’s Center for International Communication, which handles inquiries from the international news media, did not immediately respond to a request for comment.

But Saudi officials have argued that an iron fist is necessary to maintain control as Prince Mohammed oversees vast changes, like dismantling rules that yoked women to male guardians. In a 2018 interview, Prince Mohammed called the arrests a “small price” to “get rid of extremism and terrorism without civil war.”

Many of the religious figures imprisoned by the Saudi authorities hold similar views to the elder Mr. al-Ghamdi, who has criticized corruption and political repression while speaking out against elements of the social changes, including efforts to rewrite the Islamic studies curriculum in schools to remove content that the government designated as extremist.

Recently, he lambasted a performance in Riyadh by Iggy Azalea, the Australian musical artist, in which she accidentally split open her bodysuit while rapping the lyrics “Preaching about prophets/ it ain’t no one man can stop us/bow down to a goddess.”

“The perversion has reached extremes that are not possible to imagine,” the elder Mr. al-Ghamdi wrote on X.

Plenty of Saudis have celebrated the marginalization of such views under Prince Mohammed — arguing that ultraconservatives dominated the country for too long and smothered their personal freedoms. At the same time, many Saudis express concern about the speed of social changes and discomfort with what they view as the erosion of their country’s Islamic identity.

The prosecution’s case against the younger Mr. al-Ghamdi appeared to be built around anonymous accounts that he maintained on X and YouTube and his following and sharing posts from prominent Saudi dissidents.

In a handful of original posts, he criticized the Saudi royal family and other Arab leaders, saying that they were all “Zionists on the inside.” In a confession attributed to him by the prosecutors, he also indicated support for the Muslim Brotherhood, an Islamist group that the kingdom has designated as a terrorist organization, and said that the king and crown prince were “loyal to the infidels and agents of the Jews.”

The judges convicted him of charges including disloyalty to the kingdom’s rulers, “challenging the piety and justice of the king and crown prince” and “supporting terrorist ideology.”

Saudi officials sometimes argue that when they imprison religious conservatives, they are simply fighting the extremism that Western critics have long accused the kingdom of spreading.

“When they speak, you tell us they’re preaching hate,” Adel al-Jubeir, a senior government minister, said in an unusually candid lecture in 2018. “When we put them in jail, you tell us, ‘Why did you stop them from preaching? You took away their freedom of speech.’ It’s a ‘damned if we do, damned if we don’t’ situation.”

Rights groups say that the kingdom’s counterterrorism and anti-cybercrime laws are so broadly worded that they are wielded to silence many forms of peaceful dissent. Saudis imprisoned in recent years include leftist intellectuals, feminist activists and others who simply criticized government policies. Under Prince Mohammed, the rate of executions has also sharply increased, two rights groups wrote in a report this year.

The elder Mr. al-Ghamdi said he believed the authorities were trying to punish him through his brother’s case.

Under Prince Mohammed, the Saudi opposition in exile — once mainly Islamist in orientation — has grown increasingly diverse, vocal and well organized. The state has struggled to silence dissent abroad, sometimes turning to new tools.

Many exiles say that their family members back home are barred from traveling abroad. Others, like the elder Mr. al-Ghamdi, say they have faced pressure to return to the kingdom, where they fear they would face imminent arrest. He urged “all free people in the world” to try to save his brother and other prisoners in Saudi Arabia.

“My brother is unknown. He’s not famous. He used anonymous accounts to write posts, and his followers could have been counted on two hands,” he said. “But they arrested him and put him in solitary confinement for a period of almost four months.”