The King County sheriff's office says an officer arrested the 39-year-old woman Monday night after seeing her on a street and noticing blood on her clothing.
The baby is full term, but she only weighs just under 5 pounds.
When she was brought to Northwest Hospital, she needed treatment for a low body temperature and some respiratory problems, but doctors say the baby is improving and is in stable condition.
The baby is just one day old, but she may already have a history of abuse by her mother.
"Unfortunately, (the mother) has a very bad drinking problem," said Officer Beth Levin with the Shoreline Police department. "She's an alcoholic, she's usually intoxicated when I contact her. I usually find her out in the woods somewhere drinking."
Officer Levin has had plenty of run-ins with the suspected mother during the past six years.
"I saw her last week out in the woods drinking beer," Levin said. "She had a lot of clothing on, and I couldn't tell (that she was pregnant) but another officer who was with me asked her 'Are you pregnant?' and she said, 'No, I just have a lot of things in my pocket.' "
Monday night, witnesses saw a woman walk into the restroom of a Taco Bell on Highway 99 in Shoreline. Minutes later, customers heard the cries of the baby.
They discovered the newborn girl, alone on the floor of the bathroom stall. After customers described the woman to police, Officer Levin spotted her a couple of hours later.
"I saw blood on her T-Shirt, blood on the back of her shorts," Levin said.
The baby will be tested for drugs, but there are no tests for Fetal Alcohol Syndrome.
And even though the baby's suspected mother is locked up now, she has the first say over what happens to the baby next.
"We need to find out if she was using drugs or alcohol during the pregnancy," said Marry Marrs with Child Protective Services. "We need to find out what she would like us to do with this baby, if she would like this baby back -- we will work with her for that purpose."
CPS says they will even go as far as arranging a meeting between mother and baby in jail. Until then, the baby will be placed in foster care.