[2] A total of twelve teams competed in the final phase to decide the champions of the Apertura 2021 Liga MX season.
[3] For the third straight season, an additional qualifying round, the reclassification or repechaje, was employed, which expands the number of playoff spots to 12.
In the following tables, the number of appearances, last appearance, and previous best result count only those in the short tournament era starting from Invierno 1996 (not counting those in the long tournament era from 1943–44 to 1995–96).
The winners of the Reclassification matches are seeded based on their position in the classification table.
Atlas won on 4–3 penalty kicks Assistant referees:[22] Christian Espinosa Zavala (Mexico City) Michel Alejandro Morales (Mexico City) Fourth official:[22] Jorge Antonio Pérez (Veracruz) Video assistant referee:[22] Erick Yair Miranda (Guanajuato) Assistant video assistant referee:[22] Eduardo Galvan Basulto (Mexico City) There were 43 goals scored in 18 matches, for an average of 2.39 goals per match.