The United States Geological Survey says while it triggered small tsunami waves, no major destruction has been reported.
Rescue workers are searching through the rubble of collapsed buildings for survivors.
Witnesses say tremors were felt as far away as neighbouring Guatemala.
The quake's epicentre was in the Pacific, at a depth of around 70 kilometres.
The Pacific Tsunami Warning Centre says waves as high as 0.7 metres have been observed in Mexico, and issued warnings for Guatemala, El Salvador, Costa Rica, Nicaragua and Honduras.
Mexico's civil-protection agency says it's the strongest quake to hit the country since a devastating 1985 tremor that toppled buildings and killed thousands.
Most of the deaths after the earthqwuake off the nation's southern coat have come in the state of Oaxaca.
The 8.1 magnitude earthquake is the most powerful to hit Mexico in eight decades, although damage to the capital, Mexico City, has been limited.
The town of Juchitan, on Mexico's narrowest point, has been hit particularly hard.
Meanwhile, Hurricane Irma has weakened to a category-four storm as it batters the Caribbean on a path towards Florida.
The US National Hurricane Center says Irma's maximum sustained winds have decreased to near 250km/h.
The NHC says fluctuations in strength are likely during the next day or two but Irma is expected to stay a category-four storm.
