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."
A Cardiff hotel remains closed on Friday after a virus outbreak.