US and Chinese authorities are investigating the matter after the employee, who was assigned to the southern city of Guangzhou, was diagnosed with mild traumatic brain injury (MTBI), said embassy spokeswoman Jinnie Lee.
In an alert emailed to US citizens in China, the embassy said it does not know what caused the symptoms or of any similar situations in the country.
In Cuba last year, the US disclosed that 24 diplomats and their family members had fallen victim to an unsolved mysterious attack that left them with injuries resembling brain trauma. Ten Canadian diplomats and their relatives also suffered a strange illness.
"We cannot at this time connect it with what happened in Havana but we are investigating all possibilities," a US embassy official in Beijing told AFP on the condition of anonymity.
The embassy's health alert says the government employee "recently reported subtle and vague, but abnormal, sensations of sound and pressure".
"The US government is taking these reports seriously and has informed its official staff in China of this event," the message says.
"While in China, if you experience any unusual acute auditory or sensory phenomena accompanied by unusual sounds or piercing noises, do not attempt to locate their source. Instead, move to a location where the sounds are not present," it says, urging people with medical problems to consult a doctor.
Lee said the employee experienced "a variety of physical symptoms" between late 2017 through April 2018. The person was sent to the United States and diagnosed with MTBI on May 18.
"The (State) Department is taking this incident very seriously and is working to determine the cause and impact of the incident," Lee said.
"The Chinese government has assured us they are also investigating and taking appropriate measures."
The Chinese foreign ministry could not be immediately reached for comment.
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 victims suffered headaches, hearing loss, disorientation and some loss of cognitive ability.
Initially officials suspected the Americans had been targeted by some sort of acoustic weapon, although in public senior officials were more cautious, speaking of "health attacks". Media reports have suggested that the FBI has not been able to verify any evidence to support the sonic weapon theory.
The US government has held Cuba responsible, arguing that Raul Castro's authoritarian state must have either carried out the assaults or at least known who was behind them.
The incident prompted the United States last year to withdraw more than half of its personnel at the embassy in Havana, which reopened in 2015 as the two nations re-established diplomatic relations that had ruptured in 1961.
Canada announced last month that it was bringing home the families of its diplomats in Cuba after a year-long investigation into the illness failed to reveal a cause.
"The cause (of their symptoms) remains unknown but could be human-made," the Canadian government concluded.
Canadian and US authorities had initially suspected a "sonic attack" or a "mass psychosomatic incident", but those are "now considered unlikely", a senior Canadian official said.
The symptoms included dizziness, headaches and a lack of ability to concentrate.
Unlike their American counterparts, however, no Canadian envoy reported hearing any suspicious sound prior to falling ill.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)