The King County Sheriff's Office said a witness called 911 around 7 p.m. Wednesday to report a baby had been left in a locked car with the windows rolled up in the parking lot of an Issaquah shopping center, KOMO-TV, Seattle, reported Thursday.
Weather.com said Issaquah experienced highs of 85 degrees Wednesday.
Cindi West of the King County sheriff's office said the witness went into several stores, but was unable to find anyone who claimed the car or the child.
A sergeant smashed a car window to rescue the boy, who West said was lethargic after being removed from the vehicle.
The child was taken to a hospital for treatment and was released to his mother.
The boy's father, who eventually returned to the car, told deputies he had been gone for about 20 minutes.
The man was arrested on a charge of second-degree child endangerment. Child Protective Services was notified of the incident.