The sheriff's office says the 24-year-old was found unresponsive about 1 a.m. Monday and could not be revived.
She had been jailed Jan. 7 for violating a no-contact order and obstructing Everett police.
Sheriff's spokeswoman Shari Ireton says the woman had been held alone in a medical cell because of a medical condition. She had been checked every half-hour.
The sheriff's office is investigating the death, and the medical examiner's office will conduct an autopsy.
The woman's death was the eighth at the facility since 2010.
Kathleen Ann Swann-Deutsch, 51, died there in November 2013. Officials said contributing factors in her death included probable alcohol withdrawal and alcoholic ketoacidosis, an imbalance in the blood associated with problem drinking.
William Williams, 59, died in jail last September after jail staff Tased and put him in an isolation cell. In January, the Snohomish County Medical Examiner ruled the death was homicide.
Williams had been arrested that same night on suspicion of theft for allegedly shoplifting from an Everett gas station. After the death, Aaron Snell with Everett Police said Williams had become combative and wouldn't follow jail staff's commands after he was processed into jail, which led to the physical altercation.
The family of another woman who died in the Snohomish County Jail recently filed a $10 million wrongful death claim.
The claim says complacency by guards and medical staff resulted in the November 2011 death of 27-year-old Lyndsey Elizabeth Lason, who had been arrested for investigation of theft and prostitution.
Michael Saffioti, 22, died in a Snohomish County Jail cell in July of what his mother believes was a severe allergic reaction.
Despite witness testimony that jail guards mocked the dying man, the Sheriff's Office announced earlier this year it would not recommend charges against the guards or staff.