Marbella will compete in the Copa del Rey next season after a 2-1 victory in Jumilla on Sunday secured the side a seventh-place finish.
With San Fernando also winning in their final-day clash against Almeria B, only a win would have done for David Cubillos' side. The visitors raced into an early lead when centre midfielder Cruz took advantage of some sleepy defending from the throw-in to head home from just six yards out.
Jumilla, who also needed a result themselves to avoid having to play in the relegation play-off, started the game very tentatively but, ironically, the goal seemed to settle their nerves; so much so that they were the side to score the game's next goal.
Carlos Álvarez was alert to the flick-on for the long diagonal ball, controlling on his chest before firing against the post and having the presence of mind to follow in to tap home the equaliser from just in front of the goalline.
Marbella, however, were much the better side in the second half as nerves struck the hosts once again with everyone in the ground also following developments in the Don Benito-Granada and Atlético Sanluqueño-Badajoz matches.
Montero then started his own personal battle with Jumilla keeper Iván Martínez. Firstly, he had a shot from a tight angle repelled by the stopper before having a header, bound for the bottom right corner, acrobatically caught.
However, the Marbella forward was not to be denied again. With eight minutes left on the clock, he converted Jürgen's cut-back to settle the tie for good.
Cubillo, satisfied with qualifying for the prestigious knock-out competition, said: "It has been a spectacular second half of the season; we picked up 40 points - the best in the club's history. We had one objective: to finish seventh. We've done that and in doing so only conceded six goals and with just one defeat in Cartegena."
For their part, Jumilla finished in 16th place and must compete in a play-off in order to remain in the third tier.