Twice champion Alcaraz, who claimed the trophy in 2022 and 2023, reached his third tour-level final of the season after Rotterdam and Monte Carlo.
In a rematch of their Monte Carlo quarter-final last week, where Alcaraz prevailed over Frenchman Fils in three sets, the Spaniard saved the only break point he faced to extend his winning streak to nine matches.
"Being in the final in Barcelona again means a lot... in front of my people, in front of my friends who came from Murcia (and) of course the people here in Barcelona and Spain," the 21-year-old Alcaraz said.
"It's been a really fun and great week. Let's see if tomorrow we will end the day with the trophy. It's going to be tough."
Danish sixth seed Rune dispatched Russia's Karen Khachanov 6-3, 6-2 in the other semi-final earlier on Saturday to advance to his second final at ATP 500 level.
In the quarter-finals, Rune eliminated defending champion Casper Ruud.