Clallam County prosecutor Deb Kelly, who acts as coroner, said Monday the flesh and bones had been sent to the King County medical examiner's office in Seattle to determine if the remains are human.
If the foot is human, the next step would be DNA testing to see if it matches feet found washed ashore across the border in British Columbia.
Authorities said a woman told the Clallam County sheriff's office on Saturday that she found the black, high-top shoe along the beach on the Strait of Juan de Fuca, about 30 miles west of Port Angeles.
Kelly said county investigators would meet later this week with members of the Royal Canadian Mounted Police.
Five athletic shoes containing human feet have been found along the Strait of Georgia between Vancouver Island and the British Columbia mainland since August 2007. The Strait of Juan de Fuca separates the Canadian island and Washington's Olympic Peninsula.
A sixth foot found in June in British Columbia was determined to be an animal paw that had been shoved inside a shoe as a hoax.
"We're a little apprehensive since the last one was a hoax," said Detective Sgt. Lyman Moores.
DNA testing linked one of the Canadian feet to a depressed man who went missing a year ago. Investigators have also concluded that two of the five feet belonged to one man and that one foot was from a woman.
British Columbia coroner Jeff Dolan has said there was no evidence the feet were severed. Experts say that when a human body is submerged in the ocean, the arms, legs, hands, feet and head usually come off the body.