Segunda División B is the third level of the Spanish football league system. It is administered by the RFEF. It is below the top two levels of the league, the Primera División (also known as La Liga) and the Segunda División, and above the Tercera División. The Segunda División B includes the reserve teams of several La Liga and Segunda División teams.
The term Segunda División B was first used in 1929. It was used to designate a third level of teams after the Primera División and a Segunda División A. This division featured 10 teams and at the end of the season Cultural y Deportiva Leonesa were crowned champions. However the 1929–30 season saw the first of many reorganisations of the Spanish football league system and the original Segunda División B was replaced by the Tercera División. At the start of the 1977–78 season the Segunda División B was revived, replacing the Tercera División as the third level. Initially the division consisted of only two groups. The 1986–87 season was played as a single group of 22 teams. It was changed the next year, with 80 teams in four groups from the 1987–88 season.
Segunda División B currently features 80 teams divided into 4 groups of 20. The top four teams from each group, 16 teams in total, qualify for play-offs to determine which four teams will replace the four teams relegated from the Segunda División. However reserve teams are only eligible for promotion to the Segunda División if their senior team is in the Primera División. The top five teams from each group and best two teams regardless of group outside the previous twenty, excluding reserve teams, also qualify for the following seasons Copa del Rey. The bottom four teams in each league are relegated to the Tercera División. Also, the four 16th-placed teams enter into a relegation playoff to determine the two teams to be relegated. One team is paired with one of the others in home and away series. The two winners remain in the division while the losers are relegated. A reserve team can also be relegated if their senior team is relegated from the Segunda División. Along with teams from the Tercera División, teams from the division also compete in the Copa Federación.
Since the 2008-09 season, the four group winners had the opportunity to be promoted directly and be named the overall Segunda División B champion. The four group winners are drawn into a two-legged series where the two winners are promoted to the Segunda División and enter into the final for the Segunda División B championship. The two losing semi-finalists’ enter the playoff round for the last two promotion spots.
The four group runners-up are drawn against one of the three fourth-placed teams outside their group while the four third-placed teams are drawn against one another in a two-legged series. The six winners advance with the two losing semi-finalists’ to determine the four teams that will enter the last two-legged series for the last two promotion spots. In all the playoff series, the lower-ranked club plays at home first. Whenever the is a tie in position (like the group winners in the Semi-final Round and Final or the third-placed teams in the first round), a draw will determine the club to play at home first.