Crime reporter Francisco Romero, who was enrolled in the Mexican government's protection program for journalists and human rights activists, was found dead in a pool of blood in his hometown of Playa del Carmen, according to prosecutors in the eastern state of Quintana Roo and AFP reporters at the scene.
"The state prosecutor's office has opened a homicide investigation," it said in a statement.
"The victim had filed a complaint on April 12 over threats he had received."
Romero ran a Facebook-based news site called "Ocurrio Aqui" (It Happened Here) that covers local politics and crime and has more than 17,000 followers. He also worked for one of the state's leading newspapers, Quintana Roo Hoy.
"He was an independent reporter, and had been under the protection program for journalists since 2018," said Balbina Flores, Mexico director for watchdog group Reporters Without Borders.
The government program provides different kinds of protection to threatened journalists and human rights activists, ranging from panic buttons to home surveillance to bodyguards.
Flores said Romero had been enrolled in it after the murder last year of one of his collaborators, fellow Playa del Carmen journalist Ruben Pat.
Four journalists have now been murdered in the state of Quintana Roo in the past year, and five across Mexico so far this year.
Mexico is one of the most dangerous countries in the world for journalists: more than 100 have been murdered here since 2000, amid a wave of violence linked to drug trafficking and political corruption.
The vast majority of those killings remain unpunished.
Reporters Without Borders ranks the country as the third most dangerous in the world for the press, after war-torn Afghanistan and Syria.