According to Fox News, Robert Bell Wallace,67, and his wife stayed at the Hard Rock Hotel and Casino Resort on April 11 in Punta Cana.
His mysterious death comes as three other US tourists were found dead in their rooms in another hotel on the Caribbean island.
Nearly 180,000 Brits visit the country for a sunshine break every year, but the biggest market for tourism is by far the US with more than 2m Americans visiting the island.
Miranda Schaup-Werner, 41, from Pennsylvania, died on May 25 after she had a drink from her minibar at the Bahia Principe La Romana.
Five days later an engaged couple from Maryland, Edward Holmes, 63, and Cynthia Day, 49, were also found dead in their room.
Dominican authorities said post-mortem examinations revealed they all died of natural causes.
But the families are searching for answers.
Mr Bell Wallace passed away while visiting the country for his step-son’s wedding.
His heartbroken niece, Chloe Arnold, told Fox News: “He was fine.
The resort apologised and offered her and her boyfriend a meal and a massage, according to reports.
In a statement on the Grupo Pinero website the company that owns the hotel said: “In regards to the two unfortunate events that took place in the Dominican Republic, and after inaccurate and false information has been spread, Bahia Principe Hotels & Resorts would like to express our deep respect to the authorities and the ongoing investigations.
“We reiterate our firm commitment to collaborating completely with the authorities and hope for a prompt resolution of their inquiries and actions and will not be making any further statements that may interfere with them.