The 2017–18 Segunda División play-offs were played from June 6 to June 17, 2018 and determined the third team which would be promoted to the top division. Teams placed between 3rd and 6th position excluding reserve teams took part in the promotion play-offs.

Valladolid achieved promotion.

The regulations were the same as the previous season: in the semi-finals, the fifth-placed team faced the fourth-placed team, while the sixth-placed team faced the third. Each tie was played over two legs, with the team lower in the table hosting the first leg.

The team that scored more goals on aggregate over the two legs advanced to the next round. If the aggregate score was level, the away goals rule was applied (i.e., the team that scored more goals away from home over the two legs advanced). If away goals were also equal, then thirty minutes of extra time would be played. The away goals rule would again be applied after extra time (i.e., if there were goals scored during extra time and the aggregate score was still level, the visiting team advanced by virtue of more away goals scored). If no goals were scored during extra time, the winner would be the best positioned team in the regular season.^[1]