Backing the effort are Oscar-winning performers Sean Penn, Cher, Susan Sarandon, Jennifer Hudson, Gwyneth Paltrow and Charlize Theron as well as TV stars Ellen DeGeneres and William Shatner, and many others.
The Oscar-winning 2009 documentary "The Cove" chronicled the dolphin roundup in Taiji and helped spark protests over the annual hunt and ensuing slaughter. Japanese law allows a hunting season for dolphins, and fishermen defend it as a tradition.
In a letter dated Wednesday that included dozens of names, hip-hop producer Russell Simmons asked the U.S. ambassador to Japan, Caroline Kennedy, to urge Obama not to sign the Trans-Pacific Partnership, or TPP, until Japan bans the hunt. Kennedy recently tweeted that she was deeply concerned about the dolphin hunt, which has drawn widespread news coverage.
Simmons' letter said those signing don't oppose the TPP but seek to make stopping the dolphin hunt a key factor in negotiations. The free trade agreement is being negotiated by 12 nations that account for about 40 percent of global gross domestic product.
The letter said that corporations have spent the past two years crafting language in the TPP "to serve their interests."
"Should human compassion not be afforded the same privilege as business interests?" the letter stated. It added: "The world is looking to you, Ambassador Kennedy, and to our government to send a clear message to Japan that this atrocity must be banned now."
After Kennedy's tweet, a State Department spokeswoman told reporters that the U.S. was "concerned with both the sustainability and the humaneness of the Japanese dolphin hunts."
Simmons said more than 600 dolphins have been slaughtered since the hunting season began Sept. 1. Anti-hunt activists reported that dozens of fishermen helped to herd about 250 dolphins into a cove one day last month. Of those, about 40 were eventually killed for their meat. At least 50 others were kept alive for sale to aquariums and others, and the remaining dolphins were released.