The slow-moving storm - dubbed Super Typhoon Mawar - delivered rainfall of up to 5cm per hour overnight, the US National Weather Service said in its forecast.
Mawar could produce landslides, flash flooding and life-threatening storm surge, the NWS said in a series of warnings for the tropical island.
"I'm very worried for our people's safety and very concerned," Guam's Governor Lou Leon Guerrero told National Public Radio during an interview on Wednesday.
She compared the storm with 1962's Typhoon Karen, which flattened much of the island, which has a current population of about 170,000.
Typhoon Mawar's wind speeds placed the storm in category four, the second-strongest designation on the Saffir-Simpson Hurricane Wind scale.
According to early reports, Mawar damaged houses and forced the rescue of eight people, said Guerrero, who expects a more complete assessment of the destruction after winds tamper down.
Video footage and photographs posted on social media showed ominous clouds drifting over beaches, rains lashing buildings and winds bending palm trees.
"Lots of us have relocated to the basement. All the units totally flooded, several windows blown out, and the building is shuddering from the wind," Twitter user Ginger Cruz said, posting a video of winds overturning a pick-up truck outside of her home.
On Tuesday night, US President Joe Biden approved an emergency declaration, authorising federal assistance for Guam, where the United States has a military presence.
