The quake struck in the Pacific Ocean about 35 miles (55km) south-west of the city of Mapastepec, near Mexico's border with Guatemala.
The tremor was felt hundreds of miles away in El Salvador's capital, San Salvador.
There were no early reports of major damage or casualties.
The US Geological survey said on its website that the quake struck at 12:27 local time (18:47 GMT) at a depth of 40 miles.
Residents reported feeling the impact of the quake all across the region.
Mexico lies on the so-called Pacific Rim of Fire, where continental plates meet, and is regularly shaken by tremors.
