Evan Gershkovich makes his first court appearance in Moscow, where a judge upheld his detention.

Evan Gershkovich, the Wall Street Journal reporter arrested in Russia last month, appeared in good spirits as he stood in a Moscow courtroom on Tuesday. He smiled at fellow journalists he recognized. He nodded when one of them, Vasily Polonsky, called out to him to persevere and passed on greetings from colleagues and friends.

It was the first time Mr. Gershkovich, a 31-year-old American, had been seen clearly since he was detained on March 29 while on a reporting trip in the Russian city of Yekaterinburg and accused of espionage, a charge the United States and his employer have strenuously denied. At the hearing, a judge denied his appeal to lift his pretrial detention, meaning he will remain in Russian custody for the time being.

Outside the courtroom afterward, his legal team said that they had asked the judge to release Mr. Gershkovich from Moscow’s notorious Lefortovo Prison and place him under house arrest. Tatiana Nozhkina, one of Mr. Gershkovich’s lawyers, said that his legal team would continue to appeal his detention. “He asserts that he is not guilty,” she said.

Another lawyer representing him, Maria Korchagina, said that Mr. Gershkovich is ready to “assert his right for free journalism” and is “ready to defend himself.”

The case against Mr. Gershkovich has brought relations between the United States and Russia to a new low. The Biden administration has asserted that he is “wrongfully detained” — which means that the U.S. government sees him as the equivalent of a political hostage held on fabricated charges — and called for his immediate release. If convicted, Mr. Gershkovich faces up to 20 years in a Russian penal colony.

Speaking from the steps of the Moscow City Court building where the hearing took place on Tuesday, the U.S. ambassador to Russia, Lynne M. Tracy, said that it was “troubling to see Evan, an innocent journalist held in these circumstances.” She said that the “charges against Evan are baseless” and added that the U.S. government is “calling on the Russian Federation to release him.”

A day earlier, Ms. Tracy met with Mr. Gershkovich at the Lefortovo Prison, the first consular visit since he was detained and followed repeated calls from the State Department for Russia to grant access to him. He was expected to be sent back to the prison after the hearing. It is one of the most infamous detention centers in Russia, where inmates are reportedly held in intense isolation with only rare visits by lawyers.

Mr. Gershkovich’s case, the first time a Western journalist in Russia has been charged with espionage since the Cold War, has prompted an outpouring of support from colleagues, press freedom groups and international officials.

On Monday, Linda Thomas-Greenfield, the U.S. ambassador to the United Nations, condemned Mr. Gershkovich’s detention in a statement made on behalf of dozens of countries. “We urge Russian Federation authorities to release those they hold on political grounds, and to end the draconian crackdown on freedom of expression, including against members of the media,” she said.

U.S. officials are concerned that Mr. Gershkovich’s case appears to signal an even more severe Kremlin crackdown on independent news media outlets and the free flow of information within the country. While the Kremlin spokesman, Dmitri S. Peskov, has repeatedly claimed that Mr. Gershkovich was caught “red-handed,” the Russian authorities have not provided any evidence to support the accusations.