Ledecky clocked in at 8 minutes, 12.57 seconds, winning the women’s 800-meter freestyle.
Ledecky, 24, of Washington, finished 1.26 seconds ahead of rival Ariarne Titmus of Australia.
Italy’s Simona Quadarella won bronze, edging Team USA’s Katie Grimes.
Ledecky, who now has 10 Olympic medals, including seven golds, told reporters after the race that she plans on competing at the 2024 Olympics.
“That was not my last swim. I’m at least going to ’24. Maybe ’28, we’ll see,” she said.
“I just knew it was going to be my last swim here. You never take anything for granted. You never know if you’re going to be at the next Olympics,” she added.
Dressel, meanwhile, won gold in the men’s 100-meter butterfly, touching in 49.45 seconds, good for a new world record.
Hungary’s Kristof Milak finished 0.23 seconds behind, followed by Switzerland’s Ponti Sui.
“I am smiling finally. It was hard, but I had a good race against such noble rivals,” Milak told reporters. “This is a fair result. That was the maximum I could give. I am happy that Caeleb needed a world record to beat me.”
Dressel, 24, of Florida, has three medals at the Olympics, with more events to go.
Team USA didn’t make the podium in the other two swimming finals early Saturday, the women’s 200-meter backstroke and a mixed 4 x 100-meter medley relay.
America is tied with China at 45 total medals but is third in gold behind both China and Japan.