The Los Angeles County Fire Department made the discovery after responding to a medical call at Orchard Village Road and Lyons Avenue.
The collection box is located near the back of a CVS Pharmacy store.
"Her body was partially in and out [of the box]," said Natalie Arriaga, a public information officer for the Los Angeles County Sheriff's Department. "It is believed to have occurred sometime [Thursday] morning."
According to the sheriff's department, the woman was about 60 years old and was likely a resident of the Santa Clarita Valley.
Her identity has not yet been released.
Investigators said there are no signs of foul play and there are no suspects who are wanted.