The promising forward has agreed a long-term deal with the Portuguese giants after deciding to leave the Blaugrana
The 19-year-old reportedly chose the Portuguese outfit, who finished second in the Primeira Liga last term, ahead of other interested parties from Spain, England and Italy.
Cornejo spent last season playing for Barca's Under-19 side in the NextGen series, against the likes of Celtic, Manchester City and Ajax.
Under the tutelage of Benfica boss Jorge Jesus, Cornejo will hope to graduate to first-team football next season, although the Spaniard will start the new campaign with Benfica's 'B' side in the Portuguese second tier.
Cornejo confirmed his transfer on a social networking site on Thursday.
"I can announce that I am in Lisbon and am a new player of Benfica. I hope to succeed on this new stage."