LESS THAN a month after the death of Otto Warmbier, news emerged of another American student imprisoned by a hostile power. Xiyue Wang, a graduate student and researcher at Princeton University, has been sentenced to 10 years in an Iranian prison on charges of espionage. He was visiting Tehran to research the cultural history of the Qajar dynasty, which lasted from 1794 to 1925. According to his adviser, this prompted him to review and scan historical documents housed in Iran’s national archive. He was subsequently accused of “infiltration” and collecting “confidential articles” to send to the State Department and Western academic institutions. He is one of at least four American citizens and two permanent U.S. residents currently behind bars in Iran.

Mr. Wang joins Karan Vafadari, a U.S. dual citizen who owns an art gallery in Tehran, and Baquer and Siamak Namazi, a father and son who are being held in the notorious Evin prison. Baquer Namazi, an 81-year-old who had undergone triple bypass surgery, was arrested after he flew to Tehran in January 2016 to try to secure the release of his son. Another American, Reza “Robin” Shahini, was sentenced to 18 years in prison last year but is appealing the decision while free on bond. All were accused of national-security-related crimes on the flimsiest of evidence.

Iran has a long history of imprisoning Americans without due process or transparency. But Mr. Wang’s case is the first to come to light after Iran’s presidential elections in May, which resulted in a landslide victory for President Hassan Rouhani and his relatively moderate platform. On the heels of the arrest of Mr. Rouhani’s brother Hossein Fereydoun this month, news of Mr. Wang’s sentencing sends a troubling signal that hard-liners in the judiciary and Revolutionary Guard Corps. are the true nexus of power in the country.

The arrests warrant a strong diplomatic response, and early signs have been encouraging. The State Department announced new sanctions on Iran on July 18, specifically citing the detainment of Americans on “fabricated” charges as a factor in its decision. The White House also issued a strong statement condemning the arrests. Senior officials — including U.S. Ambassador to the United Nations Nikki Haley and deputy national security adviser Dina Powell — have likewise been engaged. In June, they successfully urged U.N. Secretary General António Guterres to send a letter to Mr. Rouhani calling for Baquer Namazi’s release. This pressure must be maintained until Iran releases all Americans it has arbitrarily imprisoned.

After the tragic case of Mr. Warmbier, the Trump administration has indicated that it will make freeing American hostages a priority. Addressing Iran’s practice of de facto hostage-taking is a good place to start.