The skull with a fake eye and several teeth still attached was discovered by an employee on Tuesday who informed the authorities as they feared it was related to a crime.
Police said officers responded to the reports of skull discovery and took possession of what was believed to be “an actual human skull”.
The box was dropped off at some point over the weekend, Goodyear Police spokesperson Lisa Berry said. It “did not ever make it to the floor. It was in the donation box, again, with other taxidermy items”, Ms Berry told ABC15.
"This is an unusual one," she added.
A Goodwill spokesperson said the employee “followed protocol” by calling the police immediately after discovering the skill. “Goodwill is cooperating with the appropriate authorities,” the spokesperson said in a statement.
The box was placed into a store’s bin without a return address, according to authorities.
The skeletal remains were sent to the Maricopa County medical examiner’s office where the officials found the skull to be of "historic origin" dismissing any possibility of foul play.
"It appears to be historic, ancient and does not appear to have any forensic value at all,” said Ms Berry. “And what that means is that no crime is attached to this skull.”
She praised the employees who alerted authorities, saying: "Goodwill did do the right thing. Anything suspicious should always be reported."
Sydney Steele, a Goodwill shopper, KPHO-TV that they were shocked to hear about the news of the skull discovery.
“I’ve never heard of anything donated like this, especially here,” she said. “I come to this Goodwill all the time and I’ve never heard of anything that crazy being donated.”