Michinoku Farm, a Tokyo-based company, is offering chews made from North Atlantic fin whales on its company website, with the meat described as a "low calorie, low fat, high protein" snack.
Japanese campaign group IKAN said selling products made from endangered species as treats for pampered pooches was the worst kind of conspicuous consumption.
"The most likely reason for shops to sell the whale meat dog treat is to target affluent Japanese who want to show off their wealth with something different," said Nanami Kurasawa, executive director of the pressure group.
Michinoku's website, which also sells pet goodies it says are made from Mongolian horses and kangaroos, has three different sized packets of whale chews, with a 60 gram (2 oz) bag selling for 609 yen ($5.97) and a 500 gram bag for 3,780 yen.
IKAN was one of four campaign groups that issued a joint statement on the treat.
"The product description identifies the meat as being fin whale of Icelandic origin," the statement said, adding: "Its use in pet food suggests that new markets are being explored."
"As Iceland prepares to hunt over 180 fin whales in 2013 for this export market, NGOs question the environmental and economic logic of using meat from an endangered species for the manufacture of dog treats."
Japan hunts whales under a loophole in an international ban, insisting it is carrying out research. Iceland openly defies the ban.
While whale meat is declining in popularity in Japan, many Japanese see the campaign against whaling as a symbol of cultural imperialism from the West and argue that it is a long-standing tradition.