The explosive was found in a box of dishes that was dropped off at the donation centre at 137 Avenue and 127 Street around 2 p.m.
"Volunteers opened up the box and went digging through the dishes ... And mixed within that was a grenade,” said Laurie Pratt, director of marketing for Goodwill.
Pratt said that the volunteers immediately evacuated the building and called police. The area was secured and police disposed of the grenade.
The box was dropped off by an "elderly gentleman," Pratt said.
She added that it was not the first time that weapons and other dangerous materials have been dropped off at the company and that she thinks including the grenade was likely a mistake.
"There's no way that I think ... anyone intended any harm," she said. "We have to believe no one does this on purpose."
Police believe the grenade was probably left in the box by someone who used it for military training in the past.