More than 250,000 people gathered for the Mass in St Peter's Square, which was adorned with olive trees and branches for the occasion.
The decoration for the altar was made up of 60,000 olive branches mixed with grasses and peach leaves, thyme, myrtle, ferns, strawberries, and lilies.
In his homily, Pope Francis said Jesus came for the humble, poor, and "those who do not matter in the eyes of the world".
Reflecting on the praising crowds that greeted Jesus as he entered Jerusalem, the Pope urged believers to live with the joy of salvation.
"Do not be men and women of sadness: a Christian can never be sad. Never give way to discouragement," he said.
"Ours is not a joy that comes from having many possessions, but it comes from having encountered a person, Jesus, who is among us.
"It comes from knowing that with him we are never alone, even at difficult moments, even when our life's journey comes up against problems and obstacles that seem insurmountable, and there are so many of them."
Turning his attention to young believers, he told them to "tell the world that it's good to follow Jesus".
He concluded with a call to all believers to look to the foot of the Cross and kindle the "enthusiasm of the young heart with which we must follow him during this Holy Week and throughout our lives".
Following the Mass, two centuries-old olive trees that had been placed at the foot of the statues of St Peter and St Paul were planted in the Vatican Gardens.