Asier Illarramendi Andonegi (born 8 March 1990) is a Spanish professional footballer who plays for Real Madrid as a defensive midfielder.

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 Basque 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 the 2012–13 season, as Real Sociedad finished fourth and qualified to the UEFA Champions League for the first time in ten years.

On 12 July 2013, Illarramendi signed a six-year contract with giants Real Madrid for a fee of €32.2 million, the highest amount paid by the club for a national player.