Leganes returned to LaLiga for the first time since 2020 after beating Elche 2-0 on Sunday to win Spain's second division.
The Madrid suburbs club could have gone up last weekend but fell short, eventually sealing promotion on the last day of the season by finishing ahead of the already-up Real Valladolid.
Former Brazil great Ronaldo's side Valladolid, which he is a majority shareholder in, lost 2-1 at Tenerife allowing Leganes to finish top.
Eibar, Espanyol, Sporting Gijon and Real Oviedo finished between third and sixth and will compete in the play-offs for the final promotion spot later in June.