He joins from Athletic Club Bilbao, who he joined at the age of 11.
Chelsea manager José Mourinho said: "Asier is an outstanding international player who will add quality to the squad in a position we needed to strengthen. I'm sure he will be a key player as Chelsea continues with its goal to achieve greater success both in England and in the Champions League."
Chelsea chief executive Peter Kenyon added: “José identified his target to fill the left-back position and we’re delighted that Asier has chosen to come to Chelsea. The arrival of Asier signifies Chelsea’s desire to strengthen the squad in key positions following a hugely successful season and we all look forward to seeing Asier in the blue of Chelsea in July.”
One down, two to go? Jose wanted three!