Mario Hermoso Canseco is a Spanish professional footballer who plays for Atlético Madrid. Mainly a central defender, he can also operate as a left-back. Born in Madrid, Hermoso joined Real Madrid's youth setup in 2005, aged ten, after starting at EF Concepción. On 28 September 2012, while still a youth, he appeared with the first team in that year's Trofeo Santiago Bernabéu, coming on as a second-half substitute in an 8–0 rout of Millonarios FC.
In summer 2014, Hermoso was promoted to the C-side in Tercera División, playing 34 matches and scoring two goals. On 16 July 2015, shortly after being assigned to the reserves, he was loaned to the Segunda División club Real Valladolid in a season-long deal.
Hermoso made his professional debut on 22 August 2015, starting in a 0–1 away loss against Córdoba CF. Upon returning, he was again assigned to Real's reserves in Segunda División B.
On 12 July 2017, Hermoso signed a three-year deal with RCD Espanyol. His maiden appearance in La Liga took place on 9 September, playing the full 90 minutes in a 0–5 away loss to FC Barcelona. He scored his first goal in the competition the following 28 January, but also put two past his own net in a 2–3 defeat at CD Leganés.
On 18 July 2019, with only one year left in his contract, Hermoso agreed to a five-year contract with Atlético Madrid for €25 million-plus €4 million in add-ons. Espanyol also retained 20% of any future sale of the player, with half of the fee being paid to Real Madrid. He made his league debut on 18 August, playing 28 minutes in the 1–0 home win against Getafe CF.
After playing for Spain at under-19 level, Hermoso was first called by the full side on 8 November 2018, for matches against Croatia and Bosnia and Herzegovina. He made his debut in the latter fixture, featuring the full 90 minutes in the 1–0 friendly win in Las Palmas.