Hong Kong will go for 14-day compulsory quarantine to all people arriving in Hong Kong from overseas, regardless of whether they are Hong Kong residents.
The measure comes into effect on 19 March. Ms Lam said the majority of Hong Kong's cases had been imported, adding that "strict measures" were needed. Hong Kong has seen 57 new infections over the past two weeks, 50 of which were imported, said Ms Lam.
"If we exclude these imports, we only have seven local cases in the past week," she said. "If we do not impose strict measures, our previous efforts could be wasted." Ms Lam also advised residents to avoid all non-essential travel.
There are at least 155 confirmed cases in the territory, which detected its first cases in January. The territory - a special administrative region of China - has so far been able to avoid the contagion seen elsewhere, thanks partly to a quick government response.
In January, cross-border travel with mainland China has been curtailed. Soon afterwards, health workers went on strike to demand a total border shutdown.