Diego Simeone's men offered a Hollywood ending to what has been one of the most exciting title races in recent years, with a squad lacking the global stars of both Real and Barca overcoming all the odds to land its first title since 1996.
While Diego Costa has shot to prominence as the spearhead of Atletico's attack this season, Simeone has not been able to call on the likes of Cristiano Ronaldo, Gareth Bale or Lionel Messi.
However, Simeone has proven that a combination of tactical acumen, talent and - perhaps above all - hard work can trump the mega-spending that is commonplace at the Bernabeu and Camp Nou.
That is not to say Simeone has a bunch of also-rans at his disposal - the likes of Costa, Thibaut Courtois and Koke would grace any of Europe's premier clubs - but the club has adopted a less extravagant approach than Real and Barca.
And that is why the Argentinian indicated recently that his side has become everyone's second favorite team in Spain.
The ability of Real, for instance, to sign Bale for a reported world-record fee in the offseason and for Barca to bring in Neymar shows the financial gulf Atletico has had to battle against this season.
But not only has it become the first team other than Barca or Real to win La Liga since Valencia 10 years ago, it also remains on course for an incredible double, as it faces Real in a mouth-watering UEFA Champions League final in Lisbon next week.
The prospect of Atletico denying Spanish football's two powerhouses both at home and abroad felt far-fetched when 2013-14 Liga champions Barca got its title defense off to an excellent start by hammering Levante 7-0 on the opening day of the season.
However, Atletico managed to match Barca stride-for-stride until it was beaten 1-0 by Espanyol in October, and then recovered to draw level with the Catalan giants by the turn of the year.
Real then entered the mix - taking advantage of its rivals' scoreless draw in January to move within three points of the top pair.
Carlo Ancelotti's men did not lose a league game from the start of the year until March's Clasico defeat, but from then on both it and Barca stuttered as Simeone's men closed in on the title - while also knocking Barca out of the Champions League at the quarterfinal stage.
Atletico's failure to beat both Levante and Malaga in its third- and second-to-last league games of the season set up a grandstand finale, as it visited the Camp Nou on the final day of the campaign needing to avoid defeat to end its 18-year title drought.
Barca, meanwhile, knew a win would give the club its fifth Liga crown in six years, and it was on course to do just that when Alexis Sanchez scored a stunning first-half opener.
However, Diego Godin's towering header early in the second half earned the visitors the required point - with the Camp Nou faithful even showing their gracious appreciation of Atletico's effort by applauding the opponents at the final whistle.
Regardless of whether their success proves a flash in the pan - as was the case with the fleeting success enjoyed by Deportivo and, to a lesser extent, Valencia in the early 2000s - Atletico's victory re-energizes a league that had grown slightly stale at the top.
The next test for Simeone and his players is to regularly compete with the big two - but for now, Atletico can bask in the most special and unexpected triumph of the European football season.