Marcos Alonso Mendoza (born 28 December 1990) is a Spanish professional footballer who plays as a left-back or centre-back for La Liga club Celta de Vigo.

[3] In 2008, he reached Real Madrid Castilla which competed in the Segunda División B, and first appeared for the reserve team on 22 February 2009, playing the entire 1–0 home loss against Alcorcón.

Eventually, he did not make the final list of 18, and his debut arrived on 4 April of the following year as he came on as a substitute for Gonzalo Higuaín in the 90th minute of a 2–0 away win over Racing de Santander.

[23] On 30 August 2016, after 85 games with Fiorentina, Alonso completed his move back to England after signing a five-year contract with Chelsea worth around £24 million.

[30] In April 2018, Alonso came under extensive criticism for seemingly purposely digging the studs of his boots into Shane Long's leg during a tackle in a league game against Southampton.

He was not reprimanded in any way by referee Mike Dean – who came under similar criticism – but was later charged with violent conduct by The Football Association,[31] and later issued a three-match ban.

[34] He was an unused substitute in the final of the UEFA Champions League in Porto against the same opposition (1–0 victory),[35] but started and finished both legs of the round of 16 tie against Atlético Madrid.

[40] In March 2023, Alonso's former sides Unión Adarve and Bolton Wanderers questioned the legality of the transfer and lodged complaints to FIFA over it; they alleged that they were due compensation, stating entitlement as he was on their books before the age of 23.

[5] He made 24 appearances for the eventual league champions (37 overall),[41] scoring his only goal on New Year's Eve in the 1–1 home draw with Espanyol in the Derbi Barceloní.

[44] He debuted against the latter on the 27th in a 6–1 win at the Wanda Metropolitano where he replaced Jordi Alba with 11 minutes left,[45] making the Alonsos the first Spanish family to have three generations of internationals and seventh worldwide.