About 300 of the endangered creatures were discovered by fishermen in the seaside community of Barra de Colotepec in the southern state of Oaxaca, said Heliodoro Diaz, the coordinator of the state's civil protection agency.
Video footage shows numerous corpses were partially decomposed and some already had fragmented shells.
They had been trapped in a 393-foot (120-meter) long net that is not approved for fishing.
The net has been prohibited in the area, officials said.More news: Mariano Diaz returns to Real Madrid from Lyon in five-year deal
The majority of the turtles found dead, in both instances, are Olive Ridley turtles, a species that's been endangered along the Pacific coast of Mexico since 1978.
Mexico banned the capture of sea turtles in 1990 and there are stiff penalties for anyone killing them. The office of the federal attorney for environmental protection (PROFEPA) said it would investigate the case.
On August 17, authorities said they had found 122 sea turtles dead on a beach in the neighbouring state of Chiapas, mostly from the same species.
Though the turtles' cause of death is still being investigated, experts believe the animals died from asphyxiation, toxic algae or hooks that were attached to the net, the AP reported.