With one eye on Tuesday's Champions League semi-final second leg against Liverpool, Ernesto Valverde opted to rest his big-name stars following their confirmation as LaLiga champions last weekend, including Lionel Messi and Luis Suarez.
Fran Escriba's side took full advantage of Barca's inexperienced starting XI, Maxi Gomez setting them on their way with a cool finish after 67 minutes and Iago Aspas adding a penalty two minutes from time.
The result lifts Celta up to 14th – five points clear of the relegation zone – and condemns 19th-place Rayo Vallecano to an immediate return to the Segunda Division.
Barcelona's new-look line-up suffered an early blow when Ousmane Dembele limped off with a hamstring injury after just six minutes, to be replaced by Alex Collado.
Celta did much of the pressing in the opening half but the closest they came was through Okay Yokuslu's curled effort from 25 yards, which was comfortably held by Jasper Cillessen.
The hosts thought they had forged ahead five minutes after the interval when Nestor Araujo slotted home from close range after Cillessen had fumbled Gustavo Cabral's header. Following a lengthy VAR review, however, the goal was chalked off after Araujo had strayed narrowly offside.
Moments after wastefully heading straight at Cillessen, Gomez superbly guided Ryad Boudebouz's right-wing cross past the Dutch goalkeeper from 10 yards.
Arturo Vidal was brought on by Valverde to add some bite to his lifeless team but it was Celta who had the final say, Aspas drilling home from the spot after a VAR review had deemed Moussa Wague to have handled inside the area.
What does it mean? Barca's second string fail to shine
If Valverde was hoping to witness the depth of his squad before dipping into the off-season transfer market, it looks like it may actually prove to be an expensive few months for the Catalan giants. Barca's second-string were well off the pace against a much hungrier Celta side, who were excellent value for all three points.
Gomez takes his (second) chance
The 22-year-old missed a glorious chance to put his side ahead when he headed straight at Cillessen but did not let that affect him when presented with another opportunity moments later. His finish was superb and set his side up for a famous win.
Umtiti's sluggishness sets the tone
As one of the few experienced players in the side, the World Cup winner was expected to marshal the defence. Instead, Samuel Umtiti put in a lacklustre display that explained why he has fallen out of favour this season.
Key Opta Facts
- Barcelona have lost six LaLiga away games at Balaidos in the 21st century, more than in any other venue (along with Santiago Bernabeu and Anoeta).
- Celta Vigo's Iago Aspas has scored six goals against Barcelona in LaLiga, more than any othe player since the start of 2014-15.
- Barca managed just one shot on target against Celta, their lowest figure in a LaLiga game under Ernesto Valverde.
- Celta's Maxi Gomez has scored 30 goals in LaLiga – only five players have netted more than since the start of the last last season, including his team-mate Iago Aspas (39).
- Barca have failed to score in two of their last three LaLiga away matches, as many as they had in their previous 56 in the competition.
Barca will welcome back their heavyweights for Tuesday's visit to Anfield, while Celta travel to Athletic Bilbao in LaLiga next Sunday.