Tens of thousands of people crowded El Salvador's Savior of the World Plaza for the bestowing of an honor that some wondered if Romero -- a controversial figure in his time -- would ever receive.
He was a hero to the progressive liberation theology movement, but his beatification was delayed for decades over political concerns. Pope Francis put things in motion when he declared Romero a martyr earlier this year.
"Romero, friend, the people are with you," the crowd, which numbered in the tens of thousands, chanted.
Many braved heavy rains overnight to secure a seat or spot to stand. On Saturday, the rain gave way to a blistering sunny day.
Romero was named Archbishop of El Salvador in 1977, during a period where the Central American country was run by a succession of military dictatorships.
Historians say he was chosen in part because he was seen as conservative and unlikely to be overly critical to the authoritarian government.
But the murder of a friend and fellow priest, Rutilio Grande, just one month later, brought out a new resolve in Romero.
The archbishop became an especially fierce critic of the U.S.-backed military regime that seized power in 1979.
In 1980, a group of more than 100 soldiers sent him a letter asking for his intervention regarding orders to kill guerrillas, whose ranks often included their own brothers.
In what would be his last sermon, Romero made a special appeal to the military and police: "No soldier is obliged to obey an order contrary to the law of God. No one has to obey an immoral law. It is high time you recovered your consciences and obeyed your consciences rather than a sinful order."
He concluded: "In the name of God, in the name of this suffering people whose cries rise to heaven more loudly each day, I implore you, I beg you, I order you in the name of God: stop the repression."
The next day, March 24, 1980, he was assassinated
while celebrating Mass.
"In times of difficult coexistence, Archbishop Romero knew how to lead, defend and protect his flock, remaining faithful to the Gospel and in communion with the whole Church," Pope Francis wrote in a letter Saturday to mark Romero's beatification. "His ministry was distinguished by a particular attention to the most poor and marginalized."
While his killers were never found, many blame Romero's assassination on right-wing death squads.
He was held up as a protector of the poor and marginalized who stood up to the government, though his path toward sainthood was held up for decades in a debate over whether he was killed because of his religion or because of his politics.
Pope Francis settled the issue by declaring him a martyr.
At Saturday's event, some of the faithful sold or traded Romero-related memorabilia. Everything from t-shirts to dolls to stamps.
The celebratory pop of fireworks occasionally pierced the otherwise respectful Mass. The approximately 2,000 clergy who were present helped with the massive job of celebrating communion with the masses.
Placing Romero's words in the context of today's El Salvador, where democracy has returned but gang violence is problematic, many speakers called for an end to the current bloodshed.