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.
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.
"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.
