Residents in the coffee-producing area said they heard a loud explosion before the Chaparrastique volcano began spewing gas into the air.
Authorities said approximately 5,000 people live in the San Miguel province.
The spokesman for El Salvador's Red Cross said evacuations have been ordered for inhabitants living within about two miles of the peak.
No one has been hurt, said officials.
The volcano last erupted in 1976.
Michelle Broder Van Dyke is a reporter and night editor for BuzzFeed News and is based in Hawaii.
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