Ruiz scored in the sixth minute on Wednesday (Thursday AEST) and Ousmane Dembele in the ninth following glaring mistakes by defenders Raul Asencio and Antonio Rudiger, before Ruiz made it 3-0 to cap a counter-attack.
Goncalo Ramos added the fourth in the 87th minute.
Coming off their first European title, PSG play for the championship on Sunday.
Real fared no better than Inter Milan, overrun by PSG 5-0 in the Champions League final.
The 15-time European champions looked sluggish after travelling to Florida for training between games, and PSG had 76.5 per cent possession in the first half.
A crowd of 77,542 was at MetLife Stadium on a scorching day with a temperature of 33C at kickoff and stifling humidity.
Real's Kylian Mbappe was not a threat in his first game against his former team.
Luka Modric entered in the 64th minute in probably his last match for Madrid, his team since 2012.
Eder Militao came in at the same time in his first game since tearing his right ACL on November 9.
PSG surged ahead after Lucas Beraldo's cross was mis-hit by Raul Asencio, Thibaut Courtois tipped the ball from Dembele and Ruiz slammed it into the empty net.
Dembele then took possession about 40 yards out after a Rudiger mis-hit, dribbled and slotted past Courtois.
PSG went the length of the field to make it 3-0. Achraf Hakimi exchanged passes with Dembele, then crossed for Ruiz, who maintained control despite Federico Valverde's challenge and scored from eight yards for his third goal of the tournament.
Madrid's defence was missing Dean Huijsen, who got a red card on Saturday against Borussia Dortmund, and Trent Alexander-Arnold, who had right leg muscle discomfort.
9°C