He developed at Real Sociedad, appearing in 56 competitive games with the first team and helping it qualify for the Champions League in 2013. He then transferred to Real Madrid for €32.2 million, the most they had ever paid for a Spanish player, returning to his first team for half of that fee two years later.
Illarramendi earned 31 caps for Spain from under-17 to under-23 level, winning the 2013 European Championship with the under-21 side. He made his senior debut in 2017.
A product of local Real Sociedad's youth system, Illarramendi was born in Mutriku, Gipuzkoa, and he spent the vast majority of his first four senior seasons with the B-team, contributing with 27 games and two goals in 2009–10 as the Basques returned to Segunda División B after one year out. On 19 June 2010, in the last day of the Segunda División campaign, as the first team had already sealed its promotion to La Liga as champions, he made his first official appearance, in a 1–4 away loss against Elche CF.
On 23 January 2011, Illarramendi made his top division debut, in a 1–2 loss at Villarreal CF. The following month, he first appeared as a starter playing the full 90 minutes in another away defeat (1–4 against RCD Espanyol); the bulk of the season was spent, however, with the reserves in level three.
Illarramendi started in all of his 32 league appearances in 2012–13, as Real Sociedad finished fourth and qualified to the UEFA Champions League for the first time in ten years. At the LFP Awards, he was named Breakthrough Player and Best Midfielder.
On 12 July 2013, Illarramendi signed a six-year contract with Real Madrid for a fee of €32.2 million, the highest amount paid by the club for a Spanish player. He made his official debut on 14 September by starting in a 2–2 away draw against Villarreal, and scored his first goal on 18 December in a 2–0 home win over CD Olímpic de Xàtiva for the season's Copa del Rey (also the aggregate score).
Illarramendi netted his first league goal for the Merengues on 22 February 2014, after his long-range effort against Elche was deflected and the hosts won it 3–0. His second came on 5 April, as he scored the first in an eventual 4–0 success at former team Real Sociedad in the 44th minute; he was a regular in the domestic cup campaign, coming on as a late substitute for Ángel Di María in the 2–1 final defeat of FC Barcelona, adding 11 appearances – five starts – in the team's Champions League run but remaining on the bench as they clinched the trophy against Atlético Madrid in the final at the Estádio da Luz.
Illarramendi began his second year with a brief substitute appearance in the 2014 UEFA Super Cup triumph against compatriots Sevilla FC at the Cardiff City Stadium, and was also part of Real Madrid's squad which won the FIFA Club World Cup in Morocco, playing the entirety of the 4–0 semi-final over against Cruz Azul.
On 26 August 2015, Illarramendi returned to Real Sociedad, signing a six-year deal. He made his league debut in his second spell three days later, starting and being booked in a 0–0 home draw against Sporting de Gijón. On 25 October he recorded his first goal for the club, in a 4–0 win at Levante UD.
In the 2016–17 campaign, Illarramendi had the second-highest number of interceptions across Europe's five major leagues behind only Chelsea's N'Golo Kante. However, he was also noted for his poor disciplinary record, having racked up ten yellow cards by March 2017 including one in each of his four previous games.
Illarramendi helped Spain reach the final of the 2007 FIFA U-17 World Cup, featuring in all games but one. He missed one of the penalties in the final shootout, which allowed Nigeria to lift the trophy.
Illarramendi played 16 times for the under-21s, in two years. He represented them at the 2013 UEFA European Championship in Israel, being first-choice as the nation won the competition and being selected in the team of the tournament.
Manager Julen Lopetegui first called up Illarramendi for the senior side in March 2017, ahead of a 2018 FIFA World Cup qualifier against Israel and a friendly with France. Additionally, he played for the Basque XI from 2013 onwards, scoring to open a 3–1 win over FIFA member Tunisia at the San Mamés Stadium on 30 December 2016.
Illarramendi made his debut for Spain on 7 June 2017, playing the full 90 minutes in a 2–2 friendly draw to Colombia in Murcia.As of match played 13 May 2016
1 Includes Supercopa de España, UEFA Super Cup and FIFA Club World Cup.Real Madrid
Copa del Rey: 2013–14
UEFA Champions League: 2013–14
UEFA Super Cup: 2014
FIFA Club World Cup: 2014
UEFA European Under-21 Championship: 2013
La Liga Awards: Breakthrough Player/Best Midfielder 2012–13