People for the Ethical Treatment of Animals posted the letter on its website Saturday.
Canada has asked the WTO to overturn a European Union ban on seal fur.
"As an actor who will always consider Britain home, I was proud to learn that the European Union has banned seal-fur imports," Law said. "In light of next week's seal-products dispute hearing, I'm writing to urge the panel to uphold this ban, which is in line with the wishes of compassionate people all around the world, including the majority of European citizens."
Canada holds the world's largest seal hunt. It has been controversial for decades because of film showing seals, including young ones, being clubbed to death.
Russia and the United States have banned seal imports. Most of Canada's exports go to Norway, which is not an EU member and also holds a seal hunt.
PETA has recruited a number of celebrities to oppose the seal hunt, including Pamela Anderson, the former "Baywatch" star who was born in Canada. The WTO has scheduled a hearing in Geneva.