The Barca coach celebrated his 14th trophy courtesy of an astonishing opening 25 minutes in which his side netted three times with Athletic all over the place.
He won his first trophy as Barca coach with a 4-1 win over Athletic in the Copa del Rey final three years ago, and the club's 26th victory in this final was every bit as convincing.
Guardiola embraced his assistant Tino Villanueva at the full-time whistle. Villanueva will replace Guardiola during the close season.
It did not take long for his team to get into their stride here as Pedro slipped the ball into the net from 12 yards out on three minutes after Marcelo Bielsa's side failed to clear Xavi's corner kick.
They added a second goal on 20 minutes when Lionel Messi drove the ball high into the net from an acute angle after he was picked out expertly by Andres Iniesta. It was his 73rd goal of the season.
It was more or less game over on 25 minutes when Pedro picked up a loose ball outside of the Athletic box to send a shot spinning beyond the bamboozled goalkeeper Gorka Iraizoz.
Athletic had a claim for a penalty on 27 minutes when Fernando Llorente went to ground under pressure from Gerard Pique, but the referee said no.
Messi should have added a fourth when he dashed unmarked into the Athletic box, but failed to lob the ball over Iraizoz with the goal at his mercy.
Jose Pinto made saves from Markel Susaeta and Iker Muniain in the first period, but most of the action was centred around the Athletic goal in the first half.
Athletic failed to score in their final six games of the season and their profligacy was evident in the second half with Ibai Gomez knocking an effort wide on 51 minutes and Jon Aurtenetxe missing from five yards out as a header flew wide.
Athletic's bid for a first trophy in 28 years ends in disappointment after a 3-0 loss to Atletico Madrid in the Europa League final was followed by this going over.
Barca lost the Spanish League title to Real Madrid and suffered a shock reverse to Chelsea in the Champions League last four, but this should comfort those blows.