An investigation has been launched at the Angel Hotel after management decided to shut it as a precautionary measure.
A spokeswoman said they were providing support to guests, 26 of whom have been struck down by gastroenteritis. No one needed hospital treatment.
The hotel has been contacting guests due to arrive over the next few days to help them make other arrangements.
Cardiff council said environmental health officers have confirmed the virus is gastroenteritis.
No further information can be given until the investigation has been completed, the spokeswoman added.
The council was notified on Wednesday that guests were falling ill at the hotel.
Teacher Kevin Waite from Gwaelod-y-Garth, Cardiff, said he fell ill over the weekend after attending a training event at the hotel on 5 June.
"I felt nauseous, I was having hot and cold flushes and my limbs were aching," he told BBC Wales.
"I started vomiting on Sunday and this lasted through until Monday."
Mr Waite said he has contacted the hotel to discuss his sickness and is waiting to hear from managers.
"The hotel looked plush, pleasant and clean. There was nothing to point to any problems," he added.
"I'm shocked and disappointed. But these bugs can happen for many reasons."

Police tried to arrest opposition MP Albin Kurti, who has led a series of protests in parliament against agreements made with Serbia.
Mr Kurti was reportedly not at home when masked police entered his flat in the capital Pristina on Wednesday.
Another opposition MP, Donika Kadaj-Bujupi, was arrested for letting off tear gas in parliament.
Scores of opposition supporters gathered in front of the government headquarters in downtown Pristina for a second day, throwing stones and paint at the building.
Police arrested at least 13 protesters, the AFP news agency reported.
A grenade also went off outside the constitutional court in Pristina as the raid on Mr Kurti's flat took place.
But there was no confirmation that the explosion, described by police as "controlled", was connected.
For weeks, Mr Kurti, a former leader of the Self-Determination Party, has spearheaded opposition to a deal with Serbia, mediated by the EU in August, that hands greater autonomy to Kosovo's ethnic Serb minority.
The opposition also objects to a border demarcation deal with Montenegro that it says hands Kosovan territory to its neighbour.
Police spokesperson Baki Kelani told reporters on Wednesday that they were "acting upon four detention warrants from the public prosecutor for four MPs".
He said one - Ms Kadaj-Bujupi - had been detained "while we are still looking for three others".
Kosovo, which has an ethnic Albanian majority, declared independence from Serbia in 2008 but has never been recognised by the Belgrade government.
The split came a decade after a conflict between Serb forces and Kosovan Albanian rebels. A Nato bombing campaign against Belgrade in effect forced Serbia to cede the state.
The constitutional court in Pristina last week suspended for two months the government's deal providing greater power to Serb-run municipalities pending a review. But the opposition sees the court as too close to the government.
Seven people, most of them police, were hurt during protests outside the Kosovo parliament on Tuesday. Petrol bombs and stones were thrown outside the building as pepper spray and tear gas were sprayed inside the chamber.
The violence broke out as King Abdullah of Jordan was on a visit to Kosovo.
A Cardiff hotel remains closed on Friday after a virus outbreak.
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Kosovo police have used tear gas to disperse protesters objecting to arrest warrants issued for opposition MPs.