Barca had not lost at home in the league since May 2009 when they went down 1-0 to Osasuna having already secured the title but Paraguay striker Valdez scored either side of halftime to help Alicante-based Hercules pull off one of the biggest giantkilling acts of recent seasons.
Pep Guardiola's side, whose sole defeat last term came at Atletico Madrid, looked surprisingly toothless even with Argentina forward Lionel Messi and World Cup-winners David Villa and Andres Iniesta in the starting lineup.
The reverse leaves them on three points from their opening two matches, one behind arch rivals Real Madrid.
Cristiano Ronaldo set up Ricardo Carvalho to score the only goal in Real's 1-0 win over Osasuna later on Saturday and hand their Portuguese compatriot and coach Jose Mourinho a winning home debut.
Hercules have an annual budget of around a tenth of the roughly 400 million euros Barca command but Guardiola's decision to take on the league minnows without players including Xavi, Pedro, Sergio Busquets and Carles Puyol backfired badly.
In glorious sunshine in the Catalan capital, the home side seized early control and striker Bojan Krkic fluffed a chance to put Barca ahead in the 10th minute.
The festival atmosphere for the National Day of Catalunya was spoiled when Valdez scrambled the ball over the head of goalkeeper Victor Valdes in the 27th minute.
HEAD WOUND
Guardiola threw Xavi and Pedro into the fray after the break but Hercules doubled their lead just before the hour when Valdez, who joined from Bundesliga club Borussia Dortmund in the close season, swept a Tiago Gomes cross high into the net.
Messi, Villa, Pedro and Gerard Pique, who had six stitches in a head wound after the match, all went close for Barca but the Hercules defence held firm and keeper Juan Calatayud was in sparkling form.
David Trezeguet should have made it three for the visitors in the 89th minute when he had the goal at his mercy but Valdes scrambled across to brilliantly deny the former Juventus striker.
"It doesn't get any better than winning at the Nou Camp," Valdez said in a television interview.
It was a day to forget for Javier Mascherano, bought from Liverpool last month and making his first start for Barca. The Argentina midfielder was cautioned for the foul that led to Hercules's opening goal and was hauled off at the break.
"The team was trying to the end, we couldn't do any more," Barca and Spain midfielder Iniesta told reporters.
The players would pick themselves up for Tuesday's Champions League clash at home to Greek club Panathinaikos, he added.
FIXING ALLEGATIONS
After more than a decade in the second division, Hercules had their pre-season preparations clouded by allegations of match fixing, although the club denied any wrongdoing.
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