Bilbao made life difficult for Barcelona in the first half by seating deep as the Blaugrana dominated possession.
The floodgates of goals opened in the second half when Antoine Griezmann opened the scoring with a simple finish on the hour mark.
Frenkie De Jong added the second with a header to increase Barcelona’s lead.
Lionel Messi underlined his importance to Barcelona with a solo goal before completing his brace late in the game to win another trophy for his boyhood club.
“We knew Athletic would play as they did and we had to be patient with the ball and try to create space. We were all over them in the first half and in the second they dropped off.”