Oscar Romero was murdered during Mass in 1980. It's thought he was killed by an extreme-right group as he was speaking at the chapel of a hospital for cancer patients.
Dpa International has reported that organisers expect more than 250,000 people to come and celebrate the Vatican's high honour in San Salvador.
He has been known as "Saint Romero of the Americas" for his speaking out in aid of the poor and oppressed, and his sermons often looked at social injustices in the country.
Presidents Rafael Correa of Ecuador and Juan Carlos Varela of Panama are to attend the ceremony, along with 30 international delegations.
This beatification ceremony is a step on the way to full sainthood.
He became Archbishop in 1977 and openly denounced human rights violations committed by the country's army as well as rebel groups.
It is also thought that his assassination was one of the catalysts that caused the country to enter into a 12 year civil war in 1980, which killed around 75,000 people.
The process for his beatification began in 1994 but did not take off until Pope Francis said it was important "to move in haste." He recognised his death as martyrdom in February.
Popes John Paul II and Benedict XVI said that Romero was a "martyr of the faith," but made no action to see him beatified.
