The Queen and the Duke of Edinburgh are due to fly out to Rome for a two-day visit next week.
A palace spokesman said the visit was still due to take place but a decision would be made after she is assessed in the coming days.
The Queen was due to attend a military celebration in Swansea tomorrow during the city's St David's Day celebrations.
She had planned to travel through the city to the Guildhall, where she would present St David's Day leeks to the 3rd Battalion The Royal Welsh, of whom she is Colonel-in-Chief.
She was then invited to a reception with members of the battalion and their families, before proceeding to the George Hall for lunch.
Gastroenteritis is an infection of the stomach and large intestine.
The two most common causes of gastroenteritis in adults in England are the norovirus and food poisoning, according to the NHS.
Common symptoms include vomiting, diarrhoea and dehydration.
The Queen has cancelled a visit to Swansea as she is "experiencing symptoms of gastroenteritis", a Buckingham Palace spokesperson has said.
Queen Elizabeth II was due to make the trip tomorrow.
"She will be assessed in the coming days", the spokesperson said, adding "Her Majesty is currently spending the weekend at Windsor, as usual".
The Queen has cancelled a visit due to gastroenteritis, a royal spokesman has said.