Spain winger Pedro netted in the 33rd, 47th and 72nd minute to put Barca in control and Cesc Fabregas added a fourth as Gerardo Martino's men made it five wins from five in La Liga.
Barca goalkeeper Valdes also played his part in the victory with a penalty save from Roberto Trashorras when the score was only 1-0.
That was the 11th penalty stop of Valdes' club career and the second in successive games having also kept out a spot-kick from Kolbeinn Sigthorsson during the midweek Champions League win over Ajax.
Lionel Messi was the hero of that 4-0 triumph over the Dutch with a hat-trick of his own but the Argentina superstar had to play second fiddle to Pedro today as his team-mate netted his first Primera Division treble.
Messi was still an influential force in his 250th league appearance though, and good work from him created the chance which saw Pedro slot home the opening goal in the 33rd minute.
That was tough on a Rayo side who had gone close to taking the lead a couple of times themselves, but they were handed an ideal chance to get back on terms three minutes after Pedro's strike when they were awarded a penalty following Adriano's challenge on Trashorras.
However, Valdes, who just moments before had done superbly to parry Alberto Bueno's effort, again showed his penalty-saving prowess as he dived to his right to turn away Trashorras' penalty.
Rayo would not come any closer to getting an equaliser and Barca wrapped up the victory with three second-half goals.
Pedro slotted in Fabregas' low cross to make it 2-0 in the 47th minute and then, after Neymar had struck the post, he completed his hat-trick with another first-time finish after unselfish work from the Brazilian.
Poor defending from the home side then gifted Barca a fourth goal in the 79th minute, with Fabregas clinically slotting into the bottom corner after Alejandro Galvez's attempted pass out of defence went straight to Xavi on the edge of the area.
Barca could have recorded an even more emphatic win but for the goalkeeping of Ruben Martinez while Valdes stood in the way of Rayo and a consolation goal as the Madrid-based side crashed to a fourth successive loss.