Aurelien Tchouameni's long-range strike earned Real Madrid a 1-0 victory over Copa del Rey runners-up Mallorca on Saturday to send Carlo Ancelotti's side 11 points clear at the top of La Liga.
Players boarded a barge known as the 'Gabarra' and sailed 13 kilometres down the river from the affluent town of Getxo to the Bilbao city hall, where the party continued.
Athletic have been through years of heartache, having lost their previous six consecutive Spanish cup finals and a Europa League final to Atletico Madrid in 2012.
Bilbao are the second most successful team in the history of the Copa del Rey, their 23 titles behind only Barcelona's 31, their most recent coming in 2020-21.
Sergi Darder fired the decisive spot-kick as Real Mallorca beat Real Sociedad 5-4 on penalties to book their place in the Copa del Rey final on Tuesday.
Inaki Williams raced back from Africa to face Barcelona in the Copa del Rey and sent Athletic Bilbao into the semi-finals by scoring a crucial goal in their 4-2 extra-time triumph on Wednesday.
Defeat snapped Madrid's 21-match unbeaten run.
"It's a positive result, our play was positive in many moments, and we're in the last 16," Barca coach Xavi told Movistar.
Atletico Madrid progressed with a 3-1 win at third-tier Lugo after Memphis Depay struck twice, while Real Betis were knocked out in a 1-0 defeat by Alaves.
Four-time World Player of the Year Lionel Messi has warned Barcelona's rivals that the European champions are determined to match their five-trophy haul from 2015 in the coming year.
Barcelona and star player Lionel Messi triumphed at the seventh edition of the Globe Soccer Awards in Dubai on Sunday, taking home the prizes for best team and best player.
Barcelona star Neymar said he doesn't fear passionate Real Madrid fans at the Estadio Santiago Bernabeu for the Saturday's El Clasico.
Barcelona defender Gerard Pique has hailed the selflessness of Lionel Messi, Luis Suarez and Neymar and believes the trio's lack of ego is key to their success.
Barcelona were held to a scoreless draw by third-tier Villanovense on Wednesday in a disappointing start to the Copa del Rey for the defending champion.
Injured Barcelona star Lionel Messi has failed to rule out leaving the Treble winners in the future, and has denied he is in competition with Ballon d'Or rival Cristiano Ronaldo of Real Madrid.
German football idol Lothar Matthaus has thrown his weight behind Lionel Messi to win the 2015 Ballon d'Or, insisting that no-one else merits the award.
Barcelona ace Neymar has said he wants to make history with his side as they look to build on last season's European and domestic treble.
The treble-winning FC Barcelona stars show the Champions League, La Liga and Copa del Rey trophies through the Catalan capital.
Bayern Munich coach Pep Guardiola compares Lionel Messi to Brazil legend Pele as he backs his former club Barcelona to win the Champions League.
