Tsunoda gives Japan golden start in judo at Paris Olympics
In the men's under-60kg final, Kazakhstan's Yeldos Smetov edged French hope Luka Mkheidze despite the roaring support of a packed crowd at the temporary arena on the Champs de Mars.
Japan have led the medals table at 11 of the 14 Olympics in which judo has been contested.
Tsunoda, who dropped a weight class after missing her home Games in Tokyo injured, secured gold in her first Olympics with just over a minute left in her final, rolling on her back and flipping Baasankhuu Bavuudorj head-first to the mat.
That was enough for the lesser score of waza-ari. Tsunoda, 31, had her Mongolian opponent firmly round the neck as the gong sounded.
When Smetov won silver in Rio in 2016 the feat was commemorated in Kazakhstan with a stamp showing him biting his medal.
On Saturday, he upgraded, scoring the only point by levering his opponent into the mat.
In an arena built around the equestrian statue of general Joseph Joffre, saviour of Paris in 1914, the fans chanted "Allez Luka" and sung the Marseillaise.
When the gong rang, with the Kazakh wrapped round the Frenchman, a small knot of turquoise-clad Kazakhstan fans roared.
Judo is one of the combat sports that awards two bronze medals in each event.
In the women's competition, Shirine Boukli took bronze for France's first medal of the Games.
A few minutes later 18-year-old Tara Babulfath won Sweden's first Olympic judo medal with the other bronze.
In the men's, Ryuju Nagayama of Japan and Francisco Garrigos of Spain took bronze.