He said the declaration was subject to further review.
Mr Tu’i’onetoa said Tonga has closed its borders to everyone but citizens and residents in an attempt to stop the spread of coronavirus.
Kaniva News reports the government has banned indoor gathering of more than 20 and outdoor gathering of more than 40 people.
Weddings, funerals, concerts, kava clubs, nightclubs and sports are banned.
The measures did not apply to churches and schools.
Mr Tu’i’onetoa made the announcement during a news conference Thursday afternoon.
He said all travelers from overseas will have to undergo a 14-day quarantine except doctors, nurses and other health care workers who will arrive in Tonga to help the situation.
He said there have been no confirmed or suspected cases of Covid-19 in Tonga.
The state of emergency means many closures and restrictions are now required to be followed by law.
“It is clear that this is a public emergency in Tonga. I therefore satisfy that COVID-19 is a public health emergency and is imminent and will threaten and endanger lives of people in Tonga,” the Prime Minister said.
“Therefore this required a significant and coordinated response.
“The powers under the section 30 and 37 of the Act shall be evoke to prevent and minimize illness and loss of human life.
"I therefore issue this declaration of a State of Emergency to be applied all over the land and sea areas of Tonga commencing from 20hrs of 20 until 17 April 2020, unless further review.”
Photo Tonga Government Caption: Tonga’s Prime Minister Pōhiva Tu’i’onetoa