More than 50,000 Catholics attended the service on day two of the Holy Father's trip to the country.
In his homily, the Pontiff urged young people to be a force of renewal and hope for society.
"May they combat the allure of a materialism that stifles authentic spiritual and cultural values and the spirit of unbridled competition which generates selfishness and strife.
"May they also reject inhuman economic models which create new forms of poverty and marginalise workers," he said.
Before Mass Francis met with survivors of South Korea's ferry disaster in April.
He also spoke with relatives of the dead who want a government inquiry into the sinking.
A banner outside the football ground read "please wipe the tears of the Sewol families" with a picture of the Pope.
The Pope is also meeting with young people at the shrine of the first Korean Catholic priest during Friday.