The 2020–21 Segunda División B season was the 44th since its establishment and the last as the third tier.
[1] On 6 May 2020, the Royal Spanish Football Federation announced the expansion of the league to what were initially five groups of 20 teams each for the 2020–21 season due to promotion from the Tercera División groups in the curtailed past season being applied.
[2] The season began on 18 October, following approval by each of Spain's autonomous regions' health departments.
[3] For next season the Segunda División B was demoted to the fourth level and dropped the B in its name in favour of the federation's Spanish initials RFEF on the creation of a new, two-group, 40-team third division called Primera División RFEF.
[4] 102 teams joined the league, including four relegated from the 2019–20 Segunda División and 22 promoted from the 2019–20 Tercera División.