Whitney Doughty, 25, of Sullivan was pronounced dead at the scene, the Hancock County Sheriff's Office confirmed.
Her passenger, 25-year-old Garrett Young, was taken to an Ellsworth hospital for injuries that were not considered life-threatening.
Deputies said that Doughty's ATV drifted across Punkinville Road before striking a tree.
Both riders were wearing safety belts but neither were wearing helmets, according to a press release.
The cause of the accident remains under investigation.