Two patrol ships from the environmental group Sea Shepherd found the vaquita's badly decayed carcass Tuesday, trapped in a gill net in a protected marine reserve in the Gulf of California, in northwestern Mexico, the organization said in a statement.
Environmentalists say the vaquita, the world's smallest porpoise, has been nearly wiped out by such nets, which are used to fish for another species, the also-endangered totoaba fish, which is about the same size.
The totoaba's swim bladder is considered a delicacy in Asia, where it can sell for up to $20,000 on the black market.
"If there were any reservations about totoaba gill nets being a great danger for vaquitas and other cetaceans, despite ample proof in the past, this event should definitely leave no room for doubt," said Sea Shepherd's director of marine operations, Locky Maclean.
Saving the vaquita from totoaba poachers has become a cause celebre for the likes of Oscar-winning actor Leonardo DiCaprio and Mexican billionaire Carlos Slim.
In 2017, the Mexican government launched a plan to round up all remaining vaquitas with the help of dolphins trained by the US Navy and relocate them to a protected area.
However, the project had to be aborted when one of the first captured vaquitas died.