Charles Rosenfarb, a doctor and director of the State Department bureau of medical services, said the symptoms were mixed but consistent with brain trauma.
The employee working in Guangzhou, whose symptoms of "subtle and vague, but abnormal, sensations of sound and pressure" lasted from late 2016 through to April 2018, had been sent to the USA for medical evaluation.
"The US government is taking these reports seriously and has informed its official staff in China of this event", the message says.
According to the alert issued by the US State Department on Wednesday, the cause of the injuries to the employee in China remains unknown, but officials were not aware of other similar symptoms among the diplomatic community in the country.
President Donald Trump has said he believed Cuba knew about the attacks and could have stopped them.
China's Foreign Ministry did not immediately respond to a request for comment.
US Secretary of State Mike Pompeo said that medical teams were en route to Guangzhou to investigate after an alleged sonic attack similar to those reported in Cuba. Pompeo said Wednesday he expects to have a report from the accountability review board investigating the incidents in Cuba by the middle of next week. Here, the U.S. Consulate in Guangzhou. That incident led to a reduction in staffing at the US Embassy in Havana.
Despite hundreds of billions of dollars in annual trade between them, China and the USA are considered strategic rivals for influence in Asia.
China-US relations have been strained recently, with the threat of a trade war looming.
Around November 2016, USA diplomats based in Cuba started to complain of odd ailments, including dizziness, nausea and hearing problems.
For most the symptoms occurred around May 2017, but they were first reported in late 2016.
The still-unexplained incidents sparked a rift in U.S.
Investigators have chased theories including a sonic attack, electromagnetic weapon or a flawed spying device.
Pompeo said he expects the results of an independent investigation into the Cuban incidents by the middle of next week.
"We can not at this time connect it with what happened in Havana but we are investigating all possibilities", a United States embassy official in Beijing told AFP on the condition of anonymity.
The person experienced "subtle and vague, but abnormal, sensations of sound and pressure", a statement said.
Of the 21 medically confirmed United States victims, some have permanent hearing loss or concussions, while others suffered nausea, headaches and ear-ringing. Some are struggling with concentration or common word recall, The Associated Press has reported.