Barcelona's 4-1 win over Malaga earlier meant Madrid had to win at San Mames in Wednesday's late game to seal a record 32nd La Liga crown.
They did so comfortably despite missing an early penalty from Cristiano Ronaldo with Gonzalo Higuain and Mesut Ozil giving them a two-goal lead before Ronaldo atoned for his missed spot-kick to add a third.
Ronaldo, who also missed from 12 yards in last week's penalty shoot-out defeat to Bayern Munich in the Champions League semi-final, saw his attempted dink saved by Gorka Iraizoz in the 12th minute, but within eight minutes of that miss Madrid were 2-0 up and well on their way to ending Barca's three-year reign as champions.
Higuain opened the scoring in the 16th minute with a superb 18-yard effort into the top corner, before Ozil doubled Madrid's advantage four minutes later with a far-post finish from Ronaldo's brilliant low cross.
Ronaldo got on the scoresheet himself five minutes after half-time when he headed home Ozil's corner for his 44th league goal of the campaign to all but end any hope of Europa League finalists Athletic getting back into the match.
Athletic had Spain international Javi Martinez sent off in 72nd minute as Madrid comfortably controlled the closing stages en route to the title-winning victory.
The success means Madrid coach Jose Mourinho, who is in second season at the Bernabeu, has now won league titles in four different countries having been victorious with Porto (Portugal), Chelsea (England) and Inter Milan (Italy).