In a televised press conference, President Julius Maada Bio said that a 37-year-old man who arrived in the country from France on March 16 had tested positive.
The man had been quarantined on arrival in the West African state, Bio said.
“It was not a matter of whether, but when. Ladies and gentlemen, the when is here,” the president said.
Sierra Leone had been one of the few countries in Africa to have reported zero cases, despite neighbouring Guinea and Liberia having detected infections.
There are fears that, like its neighbours, the poor nation of 7.5 million people is particularly exposed to an outbreak.
The former British colony was badly hit by the 2014-2016 West African Ebola crisis, which killed almost 4,000 people in Sierra Leone.
On Tuesday, Bio said authorities were aggressively tracing people who may have come into contact with the infected patient, and urged people to report anyone with coronavirus symptoms to the authorities.
The government had already announced anti-virus measures. Land borders are closed, international flights banned, and schools were shut from Tuesday until further notice, among other measures.
“The government will respond rapidly to changes in the situation by announcing additional enhanced measures,” Bio said.
Six of Africa’s 54 nations have been spared the coronavirus to date: South Sudan, Burundi, Sao Tome and Principe, Malawi, Lesotho and Comoros.
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