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.
Carlo Ancelotti's Madrid earned their 20th Spanish Cup in their 40th final, ending a nine-year wait to lift the trophy again
Real Madrid coach Carlo Ancelotti thinks the Copa del Rey final against Osasuna on Saturday could be his last -- although he has thought the same in the past and been wrong, he explained Friday.
Karim Benzema struck a hat-trick as Real Madrid hammered Barcelona 4-0 at Camp Nou to reach the Copa del Rey final on Wednesday, progressing 4-1 on aggregate.
A dummy dressed in a Vinicius Junior shirt was hung from a bridge near Real Madrid's training ground ahead of the Copa del Rey quarter-final clash against Atletico Madrid on Thursday.
Ousmane Dembele's strike, capping a brilliant individual display, helped Barcelona beat Real Sociedad 1-0 on Wednesday, and reach the Copa del Rey semi-finals.
Real Madrid will host city rivals Atletico Madrid in the Copa del Rey quarter-finals next week after the draw was made at the Spanish FA's headquarters on Friday.
Valencia beat Barcelona 2-1 to win the Copa del Rey on Saturday, ending the Spanish champions' four-year winning streak in the competition and denying them a domestic double.
Atletico Madrid book their place in the quarter-finals of the Copa del Rey after seeing off third-tier Lleida, while new signing Luciano Vietto's hat-trick fires Valencia past Las Palmas.
A much-changed Barcelona make light work of third-tier Real Murcia to take a commanding 3-0 lead into the second leg of their Copa del Rey last 32 tie.
Holders Barcelona have a fourth consecutive Copa del Rey final in sight when Atletico Madrid visit the Camp Nou on Tuesday with the daunting task of overturning a 2-1 semi-final, first leg deficit.
