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.
An excellent Cristiano Ronaldo free-kick couldn't prevent Real Madrid from being dumped out of the Copa del Rey as a 2-2 draw at Celta Vigo on Wednesday allowed the hosts to progress to the semifinals 4-3 on aggregate.
A fine free-kick from Cristiano Ronaldo couldn't prevent Real Madrid from being dumped out of the Copa del Rey as a 2-2 draw at Celta Vigo on Wednesday allowed the hosts to progress to the semi-finals 4-3 on aggregate.
Alaves became the first side to book their place in the semifinals of the Copa del Rey on Tuesday as a 0-0 draw at home to second division Alcorcon was enough to seal a 2-0 aggregate win.
Turkey international Arda Turan scored a hat-trick as holders Barcelona thrashed third-division side Hercules 7-0 in the second leg of their Copa del Rey last-32 tie.
Turkey international Arda Turan scored a hat-trick as holders Barcelona thrashed third-division side Hercules 7-0 in the second leg of their Copa del Rey last-32 tie.
A much-weakened Real Madrid went goal crazy in the Copa del Rey on Wednesday, smashing third-division minnows Cultural Leonesa 7-1 away from home in the first leg of their last-32 encounter.
Neymar will stay with Barcelona after signing a five-year contract, the club have announced.
Barcelona striker Luis Suarez is a doubt for next month's Copa America after hobbling off in tears midway through the Copa del Rey final win over Sevilla Sunday with a muscle injury.
A Spanish judge on Friday overturns a controversial ban on Barcelona fans flying Catalan separatist flags at Sunday's Copa del Rey final against Sevilla in Madrid.
Luis Enrique has lauded Lionel Messi as Barcelona's leader and "the best wherever he plays".
