The beatification is reason for great joy, he said, especially "for those of us who find joy in the example of the best sons of the church."
Archbishop Romero "constructed peace with the force of love, gave testimony of his faith his life," the Pope wrote.
He reminded others that God never abandons us in our difficulties.
"He sees oppression, hears the cries of pain of his children, helps to free them from oppression and takes them to a new and fertile land," the Pope wrote.
In difficult times, Archbishop Romero "guided, defended and protected his flock, remaining loyal to the Gospel, in communion with the church," he said.
His ministry, the Pope said, was distinguished for particular attention to the poorest and marginalized.
Those who have Archbishop Romero "as a friend in faith, who invoke him in as protector and intercessor, who admire him, should find in him strength and energy to build the kingdom of God," and should work for a more equitable and dignified social order, the Pope said.
"This is the moment for favourable and proper national reconciliation. The Pope participates in this hope, is united in prayer, so the seed of martyrdom is entrenched in the true paths of the sons and daughters of this nation, which prides itself on carrying the name of the divine Saviour of the world."
U.S. President Barack Obama also sent his congratulations to El Salvador.
"Archbishop Romero was an inspiration for people in El Salvador and across the Americas," he said. "He was a wise pastor and a courageous man who persevered in the face of opposition from extremes on both sides. He fearlessly confronted the evils he saw, guided by the needs of his beloved pueblo, the poor and oppressed people of El Salvador."
Obama said that "El Salvador has come a long way in the last 35 years. Those who once fought on the battlefield now compete for votes and negotiate in the National Assembly. But we, and the Salvadoran people, recognize there is much more to do."
He said the United States is "guided by the vision of Archbishop Romero. We partner with communities to promote economic opportunity, strengthen the rule of law and create safer neighbourhoods."
He also thanked Pope Francis for "his leadership in reminding us of our obligation to help those most in need, and for his decision to beatify Blessed Oscar Arnulfo Romero. Let us hope that Archbishop Romero's vision can inspire all of us to respect the dignity of all human beings, and to work for justice and peace in our hemisphere and beyond."