A US State Department spokeswoman called on China to disclose Gui Minhai's whereabouts and allow him to leave China if he wishes.
"We call on Chinese authorities to explain the reasons and legal basis for Mr Gui's arrest and detention, disclose his whereabouts, and allow him freedom of movement and the freedom to leave China," spokeswoman Heather Nauert said in a statement.
The US "will continue to coordinate with our partners, allies and regional organisations on promoting greater respect for human rights in China," Nauert said.
Gui had disappeared from his home in Thailand in 2015 and reappeared months later in China after being sentenced for a decade-old traffic violation.
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In a videotaped confession that supporters believe was coerced, Gui stated that he had turned himself in to mainland authorities over a hit-and-run accident.
That spurred suspicions that he had been abducted by the Chinese security services. He remained closely supervised even after his release from prison.
Gui was one of five Hong Kong booksellers whose disappearances symbolised China's determination to enforce its hard line on squelching political dissent and a free press despite international criticism.
He was released into house arrest in October in the eastern city of Ningbo, living in what his daughter Angela called a police-managed apartment.
The office of EU foreign policy chief Federica Mogherini has said it "fully supports the public statement and efforts of the Swedish government" on Gui's behalf.