Nine of the 14 deer that have died in Nara Park since March were found to have large amounts of plastic in their stomachs, according to the Nara Deer Preservation Foundation.
One of the animals was found to have swallowed 4.3kg (9.4lbs) of plastic, Kyodo news agency reports.
It is believed the deer are attracted to the scent of food coming from plastic bags and wrappers left in the area by tourists.
The popular park, which spans 5,000sqm, is home to about 1,200 sika deer.
They are considered sacred and have protected “national treasure” status.
Tourists flock to the area in the hope of seeing the deer bow, which they have learnt to do in exchange for food.
However, the public is warned to only feed the animals shika senbei snacks (Japanese rice crackers), which are sold in environmentally-friendly packaging from local stalls.
Vet Rie Maruko, who is a member of the preservation foundation, said he was able to feel the bones of the dead deer since they were so malnourished, Kyodo reports.
The foundation tweeted an image of a pile of plastic bags found inside the stomach of one of the dead deer with the warning: “Don’t throw away rubbish in Nara Park!”
The local government plans to do more to warn tourists not to feed the deer with anything other than the designated crackers.