The governor, Lou Leon Guerrero, said the total on the island was now 26.
The government's joint information centre said there was now evidence of community-based spread.
Ms Leon Guerrero said at least two people who had tested positive frequented the Hafa Adai Bingo Hall in Tamuning.
She said anyone who had been there between 6 and 16 of March should self-quarantine.
Yesterday, the US territory saw the Pacific's first Covid-19 related death, a 68-year-old woman.