Real reclaimed the domestic crown when their 3-0 defeat of Cadiz was followed by Barcelona's 4-2 reverse at Girona.
Only a Barca victory would have forced Real to wait.
The Madrid giants also have a chance to add to their unequalled 14 European titles.
They host Bayern Munich in the Champions League on Wednesday with their semi-final evenly balanced after a 2-2 first-leg draw in Germany.
Either Paris Saint-Germain or Borussia Dortmund will await the winners in the June 1 final.
"We deserved this league title in every way," Madrid coach Carlo Ancelotti said after guiding his team to the second league title and 12th trophy overall in his two stints with Madrid.
"We would like to celebrate with all the fans, but they understand because on Wednesday we have a very important challenge.
"We want to prepare well to make the fans happy and we will celebrate the title together on Saturday."
Ancelotti overhauled his lineup ahead of the decider against Bayern.
Centre back Nacho Fernandez was the only starter from the first leg against Bayern to begin against Cadiz.
Madrid have lost only once in 34 rounds and beat Barcelona in both league 'clasico' matches, as well as dealing Girona a pair of lopsided losses in their two meetings.
Brahim Diaz broke through for the home side in the 51st minute at the Santiago Bernabeu stadium, rifling home from the edge of the penalty area.
Season standout Jude Bellingham tapped in the second in the 68th minute, with Joselu Mato adding the third in injury time.
Barcelona took to the pitch knowing anything other than victory would leave them unable to catch the leaders.
They led 1-0 and 2-1 as Andreas Christensen and Roberto Lewandowski, from the penalty spot, struck either side of Artem Dovbyk's fourth-minute equaliser.
But Portu levelled 20 minutes into the second half and, after Miguel Gutierrez had put the home side ahead, sealed a win which sparked celebrations in Madrid.
The win lifted Girona above their near neighbours Barcelona into second place, and also guaranteed them Champions League football next season.
"We had the game under control and then gave it away, and that is a summary of our season," Barca coach Xavi Hernandez said.
"It is very tough to compete like that. We have to change many things if we want to compete next season."
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