The San Francisco man bled to death on Monday after a gray shark bit his right thigh, leaving a 15-inch wound, the Guerrero state Public Safety Department said in a statement.
The U.S. Embassy in Mexico could not immediately confirm the man's name, but local authorities identified him as a 24-year-old who was surfing with a fellow American. The other man was not injured.
The attack occurred at the Troncones beach, near the beach resort of Ixtapa.
The statement said the victim suffered wounds "that reached from the hip to the knee, exposing the femur."
The man died a few minutes after reaching the hospital from loss of blood, according to the government statement.
Shark attacks are relatively rare in Mexico.
In 2006, the International Shark Attack File at the Florida Museum of Natural History reported only one attack in Mexico, which was not fatal.