Franklin, who already holds the world record in the event, logged a time of 2 minutes, 4.76 seconds, joining fellow American Tracy Caulkins and Australian Libby Trickett for the most golds won by a woman at a single worlds.
The 18-year-old's latest medal came four days after smashing the previous world record in the 1,500- meter freestyle.
Claiming silver in the 200-meter backstroke was Australian Belinda Hocking, while Canada's Hilary Caldwell took the bronze.
Meanwhile, 16-year-old American Katie Ledecky -- who broke the previous world record in the 1,500-meter freestyle Tuesday -- set another standard Saturday.
Ledecky won gold in the 800-meter freestyle with a world-record time of 8 minutes, 13.86 seconds, finishing 2.46 seconds ahead of Denmark's Lotte Friis.