The Pedn Olva hotel said it would "fully close" for deep cleaning after discussing the outbreak with Public Health England. It is understood 13 out of around 17 staff are infected.
According to reports, the hotel had identified a coronavirus outbreak on Wednesday.
Security for the German G7 delegation and a US broadcaster, understood to be CBS, are staying there, according to Sky News.
The outbreak comes a day before the start of the G7 summit.
The hotel's owner, St Austell Brewery, said in a statement: "We can confirm that a number of our team at the Pedn Olva, St Ives, have tested positive for Covid-19.
"We immediately notified Public Health England of these cases and have been working closely with them to ensure we follow all appropriate safety guidelines.
"Following extensive discussions over the last few days with PHE and Cornwall Council, we have taken the decision to fully close the hotel.
"We fully appreciate the inconvenience given the limited accommodation options available in the area at the moment but the safety and security of our team and guests is our upmost priority. The hotel will reopen once a full Covid-19 deep clean has taken place and we have the available staff to run it."
The website for the Pedn Olva hotel in St Ives said it has temporarily shut on Thursday and directed inquiries to its owners, St Austell Brewery.