Washington said the attacks ended in the spring but now admit another attack occurred in August, making 19 that have fallen ill. Officials believe the attacks occurred in the homes of the diplomats, not at the embassy.
US Officials are calling the latest ailment a "mild traumatic brain injury," which refers to a concussion-like injury.
The announcement came late Friday, the same day the American Foreign Service Association reported permanent hearing loss is another symptom of the malady, along with brain swelling, severe headaches, loss of balance and "cognitive disruption."
US State Department Spokeswoman Heather Nauert said the administration was continuing to update its review of the scope of the attacks as new information emerges.
"We can confirm another incident which occurred last month and is now part of the investigation," she said.
The cause and source of the attacks remains unclear although Washington has said it believes the ill health affects may have been caused by a covert sonic device. Late last month Nauert said the attacks had begun in the fall 2016 and continued into the spring of 2017.
She also said at the time that 16 American diplomats in Havana had been stricken, but that the "incidents" were no longer occurring.
US officials continue to investigate the case but they remain far from a clear understanding of what transpired in the attacks, which Washington has described as unprecedented.
Cause remains a mystery
As the bizarre drama has unfolded, US officials have encouraged their diplomats to report any odd physical sensations. As a result, it remains unclear if some of the symptoms attributed to the attacks may, in fact, be unrelated.
But the latest revelation that another attack occurred just last month shows they have continued long after the Trump administration became aware of them, and raised the issue with the Cuban government. This has created more uncertainty about who may be responsible.
The Cuban government denies being behind the attacks, and Washington has not accused them of orchestrating the attacks, but it did expel two Cuban diplomats as a form of protest against Havana's failure to protect the safety of American diplomats while in Cuba.
US officials are still trying to identify what device could have caused the diplomats to fall ill. No device has been found but the state department said the attacks appeared to occur in the diplomats' homes, not at the embassy.
Former US President Barack Obama reestablished diplomatic ties with Cuba in 2014 after more than 50 years of political estrangement.
One Canadian diplomat was also stricken in March but unlike the US Ottawa has long had warm relations with Havana.
nm,bik/rc (Reuters, AP, AFP, dpa)