Police said seven men and two women were among the dead, with all the victims over the age of 18.
The crush occurred during a match between local team Alianza and the Santa Ana-based team Fas at the Monumental stadium.
The match was subsequently suspended.
The disaster reportedly occurred after a large number of fans tried to enter the venue after the gates had been closed.
Officials believe some fans had been sold fake tickets, and said that an investigation was underway.
Footage shared by local media appeared to show fans attempting to pull down barricades at the stadium's entrance.
"It was an avalanche of fans who overran the gate," a volunteer with the Rescue Commandos first aid group told journalists. "Some were still under the metal in the tunnel. Others managed to make it to the stands and then to the field and were smothered."
Luis Alonso Amaya, from the Civil Protection of El Salvador, said that about 500 people had been given medical treatment, with many transferred to the hospital.
Health Minister Francisco Alabi said that most of those injured are in stable condition and that there have been no reports of deaths from hospitals.
The Salvadoran Soccer Federation said that all national-level football matches on Sunday would be suspended. It also said it regretted what had happened and voiced support for the victims' families.
Nayib Bukele, El Salvador's president, said police would conduct an "exhaustive investigation" into what happened.
"Whoever the culprits are, they will not go unpunished," he said on Twitter.
Meanwhile, the Twitter account of Bukele's press secretary posted to say first responders were on the scene and that two victims in critical condition had been transferred to the San Rafael hospital.
Images released showed people making their way onto the pitch where they received medical treatment, while local media had earlier transmitted live footage of the incident.
El Salvador's health minister, Francisco Alabi, tweeted that the government had deployed ambulances from nearby hospitals to the stadium with the injured being sent to different public institutions for treatment.
Alabi also called for the public to cautiously evacuate the area to make it easier for medical teams to carry out care.