Paulino had been unbeaten over the distance at four consecutive meets this year and stormed to a comfortable lead halfway through the lap before finishing in 48.17 seconds on a track still wet from earlier rain showers.
The Dominican had a slower start than her rivals but demonstrated her racing prowess by reeling in the early front-runners with a powerful kick that powered her to break the Olympic record set in 1996 by France's Marie-Jose Perec.
"I don't know how to express it because there are so many emotions," Paulino, who won silver in Tokyo, said.
"It was really the only medal I needed to be one of the athletes to have conquered a gold Olympic cycle in a short time."
Paulino, who comfortably won last year's world championships, also led the Dominican Republic to a silver in the 4x400m mixed relay in Tokyo.
Bahrain's Salwa Eid Naser took silver and Poland's Natalia Kaczmarek earned the bronze.
Before the Olympics Kaczmarek had the fastest time this year amongst the eight finalists but she could not respond to Paulino on the night. Still, the Pole was delighted with bronze after not making it to the final in Tokyo.
Rhasidat Adeleke finished fourth but still made history as the first woman from Ireland to reach an Olympic 400m final.
Naser, who clocked her season's best of 48.53, said: "I'm ecstatic and overwhelmed with joy and gratitude."
Double Olympic champion Shaunae Miller-Uibo was not in the final to defend her title, as the Bahamian athlete struggled with injuries and failed to make it through qualifiers.
