California man Robert Bell Wallace, 67, died on 14 April after getting sick at the Hard Rock Hotel & Casino in Punta Cana.
The same month John Corcoran died in his hotel room, according to his sister, Barbara Corcoran, who stars on TV show Shark Tank. She attributed his death to "natural causes".
Miranda Schaup-Werner, a 41-year-old from Pennsylvania, died on 25 May after drinking from her hotel minibar, US media report.
Her body was found hours after she checked into the Luxury Bahia Principe Bouganville.
On 30 May, a hotel worker discovered the bodies of Cynthia Ann Day, 49, and Edward Nathaniel Holmes, 63, in their rooms.
Local investigators say both had internal bleeding and fluid in their lungs.
The 11 June death of Leyla Cox, 53, was confirmed by the US State Department. Her son has questioned the official ruling, which found she had suffered a fatal heart attack, pulmonary edema and respiratory failure.
Last July, David Harrison died at the Hard Rock Hotel & Casino. His wife said he had been snorkelling the day before his death.
Perhaps it's just a coincidence?
Local officials have called the deaths "unrelated and isolated", attributing them to natural causes.
The US embassy in Santo Domingo said it had not yet found anything to indicate the cases are linked.
The deaths represent a tiny percentage of the Americans who holiday on the island every year.
Seven million tourists visit the Dominican Republic annually, with nearly half coming from the US.
Tourism Minister Francisco Javier Garcia said earlier this month: "These are isolated incidents and the Dominican Republic is a safe destination."
Who else has got sick?
Dozens of other visitors also said they were sickened after staying at hotels and resorts around the country in the past year.
A group of around 40 Oklahoma high school graduates say their trip in early June was spoiled when they ate at a Japanese restaurant managed by the Hard Rock Hotel & Casino in Punta Cana, where two Americans died after getting ill.
"We just don't know what is happening," said Liz McLaughlin, whose daughter fell sick. "Is it the water? Is it the ice? Is it the food? Is it the food handling? Is it the pesticides?"
A group of 114 fans of Jimmy Buffett from Oklahoma say about half their group was sickened after a trip to the Hotel Riu Palace Macao in Punta Cana in April.
"The common thread with everybody that did get sick is that they swam in the swim-up bar pool and/or they had a drink from the swim-up bar," said Dana Flower, a travel agent for the group, the Central Oklahoma Parrothead Association.