The Nara Deer Preservation Foundation said that masses of plastic bags and snack packets were found in the stomachs of the deer which died between March and June this year.
"The biggest litter found in one of the nine amounted to 4.3 kilograms," foundation official Yoshitaka Ashimura said.
In March this year, the Nara Deer Preservation Foundation posted a picture to Twitter reportedly showing a lump of tangled plastic bags, weighing 3.2 kilograms, that was found in the stomach of a deceased deer.
Visitors to the famous park have surged in recent years, with more than 16 million people visiting the park in 2017. More than one thousand deer freely roam the park and are protected as a national treasure.
A veterinarian who belongs to the Nara deer conservation group told local Kyodo news agency that the animals have likely developed the habit of eating plastic bags due to tourists feeding them food from plastic bags brought into the park.
The deer have now learnt that the bags contain food or are tempted by the smell on the discarded packages, Rie Maruko said.
Signs throughout the park ask tourists not to feed the deer anything other than a special cracker, called shika senbei, that can be bought near the park and are packed in paper.
The clean-up crew recovered more than 30 kilograms of plastic waste from the park on Wednesday, according to a blog post from the conservation group.