Martinez underwent a medical on Wednesday before putting pen to paper on a five-year deal, bringing an end to weeks of speculation.
"This was without question a complicated transfer," said Bayern chairman of the board Karl-Heinz Rummenigge, "so we're all the more happy that Javi Martinez is finally with us. We all know he's a good player and over the past few days he's proven himself to be a really strong character who is already showing a strong kinship with FC Bayern. We - Jupp Heynckes and Matthias Sammer in particular - are very much looking forward to working with him."
Martinez himself is eager to get started having been the subject of one of the most drawn-out transfer sagas of the summer: "I'm very happy and looking forward to being at a club with such a great history. I had to wait a long time for this, but now the wait is finally over."
"I've come here to win titles"
"Bayern are one of the best teams in Europe, even the world," he continued. "There are so many top players here who all play fantastic football and that's a great motivation for me. Obviously I've come here to win titles too."
Competition for places in the Bayern midfield is already fierce, but the arrival of Martinez is a clear statement of intent from the record champions as they prepare to wrangle back the Bundesliga and DFB Cup trophies from Borussia Dortmund and go one better in this season's UEFA Champions League.
Wealth of experience
The 23-year-old midfielder made over 200 appearances for Athletic Bilbao, reaching the UEFA Europa League final last season, only to see his side go down 3-0 to Atletico Madrid in the final. Captain of Spain's U21 European Championship-winning side, the versatile midfielder was also a member of the senior squads that won the 2010 FIFA World Cup and UEFA EURO 2012.
Martinez joins former U21 team-mate Alvaro Dominguez (Borussia Mönchengladbach) in Germany's top-flight, with the likes of Schalke 04's Jose Manuel Jurado and Sergio Escudero already part of the Bundesliga's growing Spanish contingent.