Abe pointed two fingers in the air after beating Brazilian Willian Lima in the final to indicate that he now owned two Olympic gold medals, including the one he won in front of empty stands in Tokyo during the COVID pandemic three years ago.
"I had never seen that view, of the venue filled with people. And here I had my whole family watching," Abe said, adding that he would be back to defend the title in Los Angeles in 2028.
Lima, disappointed that he had missed his shot at gold, thought the ippon that gave Abe victory was not a point.
"Tomorrow I will know what I did today," he said. "You have this chance only one or two times in your career, and you have to grab that medal. And today I couldn't."
Keldiyorova had elbowed her way into the final on the back of an ippon upset of Uta Abe in the round of 16, ending the reigning champion's unbeaten run in individual competition that went back to 2019.
Abe, who had won gold within 30 minutes of her brother in Tokyo, sank to her knees holding her head in her hands as she tried to come to terms with what had happened.
In the final, Keldiyorova took an early lead over Distria Krasniqi of Kosovo and held on to win her first major global title after two silvers and a bronze at three world championships.
There were tears aplenty earlier in the women's competition, when Brazilian Larissa Pimenta and Amandine Buchard, silver medallist behind Abe in Tokyo, both won bronzes.
Buchard was roared on by a partisan crowd as she earned a third judo medal for the host nation at the Paris Games and the twice European champion was mobbed by friends and family in the stands at the Champ-de-Mars Arena.
Pimenta, who was also knocked out by Abe three years ago in Tokyo, overcame Odette Giuffrida to claim her first Olympic medal, collapsing on the mat in floods of tears as the Italian congratulated her.
Gusman Kyrgyzbayev of Kazakhstan and Denis Vieru of Moldova won the bronze medals in the men's competition.
