They were found on Arrow Mushrooms Gateforth Farm near Selby by fire crews clearing up after a blaze had destroyed derelict buildings there on Thursday.
A forensic archaeologist has confirmed the bones are from five individuals and are more than 100 years old.
Police said there were no suspicious circumstances surrounding the find.
A North Yorkshire Police spokesman said: “They have clearly been buried for a considerable time, in excess of 100 years, and all indications are that they date back to the Middle Ages.
“It is apparent that they have been transported to the location… in a consignment of soil.
“North Yorkshire Police can confirm there are no suspicious circumstances surrounding this find and it is purely an archaeological find.”
Source: BBC News