The win made up for a disappointing eighth-place finish for Miller-Uibo in the 200m final earlier in the week
Shaunae Miller-Uibo of the Bahamas won the women's 400m in 48.36s Friday to retain her Olympic title as American Allyson Felix took bronze for a record 10th Olympic medal.
Marileidy Paulino of the Dominican Republic won silver.
Miller-Uibo wrested control of the lead on the second bend and delivered her trademark final surge to become the first woman to win back-to-back Olympic titles in the event since France's Marie-Jose Perec in 1996.
She finished in 48.36s - more than eight tenths of a second faster than Paulino, her place on top of the podium never really in doubt.
It was the fastest performance of the season in the event, after Miller-Uibo set the previous season's best of 49.08 in Eugene, Oregon, in April.
The win made up for a disappointing eighth-place finish for Miller-Uibo in the 200m final earlier in the week.
Felix's bronze made the 35-year old the most decorated woman in Olympic track and field history, edging ahead of Jamaica's Merlene Ottey.
She also equalled compatriot Carl Lewis’ Olympic medal count and is widely expected to contend in Saturday's 4x400m relay final for one more shot at the podium in her fifth and final Games.
Felix and Lewis are tied for second all-time in the sport behind Finland's Paavo Nurmi who won 12 Olympic medals.
Already one of the sport's greats, Friday's performance capped a remarkable saga for Felix after she gave birth to daughter Camryn via an emergency C-section in 2018.
It was another fast event at the Olympic Stadium, where records have fallen in a dazzling athletics program, as the Dominican Republic's Paulino set a national best to finish second.