2020–21 Tercera División

The 2020–21 Tercera División was the last for this league as the fourth tier of Spanish football. It began in October 2020 and ended in June 2021 with the second phase and promotion play-off final in the Canarian group.

Because the Royal Spanish Football Federation (RFEF) suspended the previous season on 11 March 2020 at the onset of the COVID-19 pandemic in Spain, the RFEF announced on 6 May the termination of that season, the revocation of all relegations from the Segunda B and Tercera divisions, and the expansion of both leagues. Each regional federation was allowed to plan its own group for the 2020–21 season and as this season became somewhat shorter than usual, the RFEF recommended the subdivision of each region into two groups in the first phase for ease of scheduling, with a final phase in which the teams regrouped based on initial positions.

Also, the Tercera División dropped down to the fifth level and Segunda B to the fourth, with Tercera suffixing the federation's Spanish initials RFEF to its name and Segunda B replacing the B with those initials after the creation of a new, two-group, 40-team third division called Primera Federación that would begin in 2021–22.

Competition format

 * The top eligible teams in each initial group played in the promotion and play-off groups.
 * The first placers of each final group qualified for the 2021–22 Copa del Rey. If the placer was a reserve team, the first non-reserve team qualified would join the Copa.
 * In each relegation group, at least five or six teams were relegated to regional divisions.