The infection was detected in a fully vaccinated person who had arrived on a repatriation flight from New Zealand.
Tonga’s Prime Minister Pohiva Tu’i’onetoa warned that residents on the main island of Tongatapu face a possible lockdown next week.
Tonga was one of the last countries not to have reported Covid infections.
Over 100,000 people live on the island nation, located north-west of New Zealand.
Only a third of Tonga’s population have been fully vaccinated. But national immunisation co-ordinator Afu Tei said that thousands had been turning up at vaccination centres to receive their jabs.
The infected person was among 215 people on a repatriation flight from Christchurch, New Zealand. Others onboard included members of Tonga’s Olympic team who had been stranded in the city since the Tokyo Olympics.
New Zealand’s Health Ministry said that the individual had tested negative before leaving the country.
But authorities in Tonga said a positive reading was recorded after a routine test on Thursday, done while in compulsory managed isolation.
Other countries such as North Korea and Turkmenistan have yet to report any cases, but experts agree that the virus may be present, even if it is not officially confirmed. BBC