Wing Cheslin Kolbe, hooker Malcolm Marx and centre Jesse Kriel also crossed for tries as the hosts bullied Argentina in the scrum and ran the visitors ragged with inventive backline play that they have introduced into their game this season.
Argentina’s only try came via first five-eighth Tomas Albornoz and they were not helped by yellow cards for wing Mateo Carreras and fullback Santiago Carreras, and a 20-minute red card for flanker Pablo Matera as they lost their discipline under pressure.
There was double cause for celebration as lock Eben Etzebeth became the most capped Springbok with his 128th appearance, one more than Victor Matfield, as the world champions finished with 24 points in the table, eight more than New Zealand.
"I am lost for words, it has been a journey and a half and to bounce back from last week says a lot about our team," Springbok captain Siya Kolisi said.
"I want to say to my best friend, Eben Etzebeth. You have always been by my side, you have led us as a team in so many ways. I am so proud of you. Cheers my brother, I love you."
The Springboks selected Manie Libbok at first five but deferred his kicking duties from the tee to scrumhalf Jaden Hendrikse, signalling their intention to run the ball.
And Libbok, who missed a late penalty in front of the poles last week as Argentina won 29-28 in Santiago del Estero, was the conductor for the Springboks and ignited their backline as they ran in seven tries.
South Africa bagged two of those tries early to settle any nerves as they kept the visitors penned in their own half.
Fassi crossed after a backline move from the set-piece saw him bash his way over near the posts, before Du Toit stretched over the top of a ruck to dot down and make it 14-0 in as many minutes.
Argentina hit back, opening up the Springboks defence with some quick hands for Albornoz to canter in for their only try.
The visitors picked up their first yellow card of the Rugby Championship when Mateo Carreras impeded Fassi in the air.
The Springboks dominated possession with their man advantage and Fassi scored his second try as they moved the ball left and right across the pitch to work the space for him to go over.
The home side added a fourth with a superb jinking finish from Kolbe as he turned his defender inside out in the corner to dot down and make the halftime score 27-7.
Argentina lost Matera to a red card 15 minutes into the second half for a dangerous clear-out at the ruck on Vincent Koch and Santiago Carreras to a deliberate knock-down.
With their numerical advantage, the home side added further scores through Marx, Du Toit and Kriel to claim their first southern hemisphere title since 2019 and their fifth overall.
"Credit to South Africa, they were the better team today," Argentina captain Julian Montoya said.
"We weren’t good enough, we had to be better."
