Ruiz and Ousmane Dembele scored inside nine minutes to stun Real and silence the majority of the 77,542 crowd who were wearing white at the MetLife Stadium outside New York.
The Spanish midfielder added another before half-time and Goncalo Ramos got a late fourth as Luis Enrique’s side moved one game away from glory at the revamped tournament.
Victory against Chelsea on Sunday would complete a campaign of unparalleled success for the Parisians, who won the French title and then the first Uefa Champions League in their history in May.
“We are in a special season, a special moment, and we deserved to win,” Luis Enrique said before looking ahead to the final.
“We are going to try, one more step against a good team like Chelsea. They did great, they performed the whole competition and now it is time for us to prepare for that. We want to make history in our club, and then take some holidays.”
After blowing away Inter Milan 5-0, the biggest victory in European Cup final history, PSG began their Club World Cup bid by putting four past Atletico Madrid and the ease with which they disposed of Real was striking.
PSG’s fluidity contrasted starkly with Madrid, who are a work in progress under new coach Xabi Alonso.
“We were not good enough today,” said Alonso. “We are not the first ones to lose to them but we have a lot of different things to improve.”
Mbappé struggled to make any impact on his first start at the tournament and in his first game against PSG since leaving the French club last year.
The defeat is a real blow to Real as they hoped to win the first 32-team Club World Cup to add to their record tally of 15 European Cups.
PSG were without imposing centre-back Willian Pacho due to suspension following his sending-off against Bayern Munich in the quarter-finals, but that did not matter.
Lucas Beraldo slotted into the defence with ease, and the rest of PSG’s first-choice line-up was there. (AFP)