The 31-year-old international was an influential figure in his homeland, at Athletic Bilbao.
His 11-year spell at the Basque side came to an end this summer and Blues and Aitor Karanka have pulled off a coup by subsequently bringing him to the St. Andrew's Trillion Trophy Stadium on a free transfer.
San José made just shy of 400 appearances for Bilbao (37 goals) and was part of their run to the finals of both the Europa League and Copa del Rey in 2012 under Marcelo Bielsa.
Bilbao knocked out Manchester United, Schalke and Sporting Lisbon on their way to the final in Bucharest, which was won 3-0 by Atletico Madrid.
Capped seven times by Spain and a member of their Euro 2016 squad, San José plays primarily as a central defensive midfielder or centre-half.
Standing 6ft 2in tall, he is an imposing performer, athletic and excellent in the air as well as adept with the ball at his feet. San José reads and breaks up play well and boasts a fine range of passing.
He has signed a two-year contract at Blues and will wear squad number 19.
A legendary figure at Bilbao, they paid a special tribute to him in their final match at the San Mames in July. Along with Beñat Etxebarria, another long-serving player bidding farewell, he was afforded a guard of honour before a presentation made by the club president.
San José won the Under-21 European Championships in 2011, following on from European success at Under-19 level.
From Pamplona, he came through the youth ranks at Bilbao before signing for Rafa Benitez’s Liverpool aged 18 in August 2007.
He was regular in the reserves for two seasons before a loan to Bilbao in August 2009, which was made into a permanent move, so setting him on his way for an accomplished career to date.
San José won the Spanish Super Cup in 2015 when Bilbao trounced Barcelona 4-0 in the home leg - he netted the opener in the 13th minute.
And what a goal it was: he scored with stunning strike from inside the centre circle, sending Marc-André ter Stegen's clearance back over his head and into the net.
It enabled Bilbao to lift a trophy for the first time in 31 years against the Champions League holders.
He was runner-up in 2009 and also played in the Copa del Rey final of 2014/15 which was won 3-1 by Barcelona.