The Portuguese forward, who became the world's most expensive player when he joined Real from United in 2009, set up Karim Benzema to open the scoring from close range in the 18th minute at a festive Bernabeu stadium.
Ronaldo doubled the home side's lead with a powerful left-foot strike from the edge of the area eight minutes later and added his second in the 46th minute after a blunder in the Sevilla defence.
The 28-year-old completed his treble in the 59th minute when he started and finished a swift break to chalk up the 21st hat-trick of his career and his 17th in La Liga.
Ronaldo took his tally for the season to 24 league goals, 10 behind top scorer Lionel Messi of Barcelona.
"We have a phenomenon [in Ronaldo]. I cannot recall anything like it," Real director and former club great Emilio Butragueno told Canal Plus television.
"It seems normal for him to score three goals and we continue to enjoy his performances."
Real forward Gonzalo Higuain was sent off midway through the second half for a second yellow card but it did little to sour the party atmosphere ahead of United's visit in a last 16 first leg tie.
Ronaldo was substituted to a rousing ovation shortly before Higuain's dismissal and Sevilla's misery was compounded with 10 minutes left when Hedwiges Maduro picked up a second yellow.
Substitute Manu del Moral pulled a goal back for the Andalusians three minutes from time.
NICE WEEK
In the earlier kick-off, Isco followed up his midweek debut for Spain with two goals to give Malaga a 2-1 win at Levante that cemented their hold on fourth position.
The promising 20-year-old playmaker, a second-half substitute as the world champions beat Uruguay 3-1 on Wednesday, equalised with a penalty in the 36th minute and fired home a low shot for the winner a minute after half-time.
Jose Javier Barkero had put Levante ahead with a penalty in the 27th minute after a Martin Demichelis foul on Obafemi Martins.
"It has been a really nice week but we can't stop here," Isco told reporters. "We have to keep fighting because this team has a chance to qualify for the Champions League.
"We are fourth and have our fate in our own hands."
The victory lifted Malaga, who play at Porto in the last 16 of the Champions League on February 19, back above fifth-placed Valencia. Levante are ninth.
Valencia's Nelson Valdez came off the bench to snatch a last-gasp goal in a 1-0 win at Celta Vigo that give his side a boost before they host Paris Saint-Germain in the Champions League on Tuesday.
Celta held Ernesto Valverde's team at bay until the third minute of added time when Joao Pereira floated a cross to the back post and Valdez, who had only been on the pitch a few minutes, nodded powerfully past Javi Varas.
The victory came at a cost, though, as Valencia lost Aly Cissokho when the full-back went over awkwardly on his ankle and had to be replaced towards the e
