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.
The regulations are the same as the previous season: in the semifinals the fifth placed team faces the fourth, while the sixth placed team faces the third. Each tie is played over two legs, with each team playing one leg at home. The team that scores more goals on aggregate over the two legs advances to the next round. If the aggregate score is level, the away goals rule is applied, i.e., the team that scores more goals away from home over the two legs advances. If away goals are also equal, then thirty minutes of extra time is played. The away goals rule is again applied after extra time, i.e., if there are goals scored during extra time and the aggregate score is still level, the visiting team advances by virtue of more away goals scored. If no goals are scored during extra time, the winner will be the best positioned team in the regular season.
^Sporting Gijón manager Rubén Baraja was given a four-match touchline ban following the home match against Barcelona B. Assistant manager José Ramón Rodríguez filled in as manager for this game, the last one of the sanction.