The quake's epicenter was about 17 miles (27 kilometers) from Ometepec, Guerrero. It was about 7.6 miles (12 kilometers) deep, the USGS said.
There were no immediate reports of damage or injuries.
Officials described the quake as an aftershock of the 7.4-magnitude temblor which struck in the same area on March 20, damaging hundreds of homes.
There have been 280 aftershocks since that quake, Mexico's National Seismological Survey reported.
Residents felt Monday's quake in Mexico City, hundreds of miles from the epicenter. There were no initial reports of major damage in the capital, Mayor Marcelo Ebrard said on Twitter.
The USGS describes Mexico as one of the most seismically active areas of the world. The country sees an average of seven earthquakes daily with a magnitude greater than 3.0, according to Mexico's National Seismological Service.