Beth Shriever won BMX racing Olympic gold minutes after Great Britain team-mate Kye Whyte secured silver in the men's event.
Shriever had looked the class of the field in qualifying and delivered in the final.
The 22-year-old led from the first bend, holding off a late charge from defending champion Mariana Pajon of Colombia down the final straight and ended Pajon's bid for a third straight Olympic gold medal.
The former junior world champion from Leytonstone, who needed crowdfunding to make it to Japan, secured Britain's sixth gold medal of the Games.
Shriever was lifted into the air by Whyte following her race as the BMC racing pair celebrated the double triumph on their Olympic debuts.
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"I'm just in bits," Shriever said.
"I tried my hardest out there today and to be rewarded with a gold medal is honestly mind-blowing. I kept my cool today, kept it simple, and it worked. I'm over the moon. I've done my family and my boyfriend back home proud.
"I don't think it's registering right now what's just happened."
Whyte had earned Britain's first BMX racing Olympic medal moments earlier and missed out on gold by just 0.144 seconds.
The 21-year-old Londoner, having been hampered by poor starts in qualifying, made it into the first corner of the final in second place and held his position as Niek Kimmann took gold for Holland.
The two crossed the line with a healthy margin after a late crash for Frenchman Romain Mahieu split the field, with Colombian Carlos Ramirez Yepes taking bronze.
Whyte had shown in qualifying he had the speed to contend if only he could get off the line, needing to recover from poor starts in the first two runs but then winning the final one to be sure of his place in the final.
And there were no mistakes when it really mattered as Whyte quickly got himself into position.
Defending champion Connor Field of the United States was unable to contest the final after crashing heavily in the last run of his semi-final, leaving the track on a stretcher.
Whyte said: "With the way I was riding today, I was obviously catching people but I thought I would have to do the same in the final and it's very tiring.
"I just came out of the gate, and I didn't expect it but in the back of my head I kept telling myself I was going to get a medal.
"I didn't deserve it any more than any of the other riders but I put my head to it and I got a medal."
Whyte began his celebrations by a TV screen showing a link to the party taking place at the Peckham BMX Club - the place where his father was a coach, his mother the secretary, and his biking career began.
This double success, following on from Tom Pidcock's mountain bike gold on Monday, gives British Cycling quick results in their bid to look beyond the velodrome at these Games.
Neither the men's mountain biking nor the women's BMX were funded by UK Sport in this Olympic cycle, but British Cycling applied to redirect funds from elsewhere in order to back their riders - and have been rewarded with gold medals in both.