[2][3] He made his senior debut in the 2009–10 season, helping the reserves to promote from Tercera División after one year out by contributing 23 games and one goal;[4] he started out as a left-back.

[5] On 27 August 2011, Martínez made his first-team – and La Liga – debut with the Basques, playing the full 90 minutes in a 2–1 away win against Sporting de Gijón.

[8] He continued to be first choice in the following top-flight campaigns, notably helping the club to return to the UEFA Champions League after ten years in 2012–13 by netting four times[9][10][11][12] in 34 matches.

[17] During October 2018, Martínez conceded two penalties for foul challenges in two local derby matches in the domestic league, both of which led to the opening goal and were awarded after Video Assistant Referee reviews – the first in a 3–1 home defeat to his old club Real Sociedad[18] and the second in a 1–1 away draw against Eibar.

[26] His early tenure was hindered by a foot injury that required surgery shortly after his arrival, delaying his registration and relegating him to fifth-choice behind Ronald Araújo, Andreas Christensen, Eric García and Jules Koundé.

[36] Between September 2018 and March 2021 he received eleven caps, including seven starting appearances;[37][38] however, he was not included in Luis Enrique's 24-man squad for the UEFA Euro 2020 tournament (delayed for a year by the COVID-19 pandemic in Europe) when it was announced on 24 May,[39] with the player stating that he was not totally happy with his physical situation having missed a number of club matches during the season and an agreement had been made between him, the national body and Athletic that he would not take part.

[41] Martínez has been likened to compatriot Carles Puyol and is known for his heading ability which makes him an aerial threat on set pieces, despite his modest stature of 1.82 m (5 ft 11+1⁄2 in).