The 18-year-old clocked a time of two minutes 04.76 seconds, a championship record, with Australia's Belinda Hocking taking silver in 2:06.66 and Hilary Caldwell of Canada bronze in 2:06.80.
Franklin already had golds this week in 100 backstroke, 200 freestyle and the 4x100 and 4x200 freestyle relays and she came fourth in Friday's 100 freestyle.
Her fifth triumph means she has matched a feat previously achieved only by American Tracy Caulkins in 1978 and Australian Libby Trickett in 2007.
"It's hard to comprehend but I'm so excited because 200 back is my favourite race so it was so fun to get out there," Franklin told Spanish television.
Asked what her limit might be, she added: "I don't know, I don't think I can choose.
"I'm so flattered I had so many opportunities for all these races and it's so exciting to be able to swim so many times here.
"It's an honour and a privilege and I enjoy every single time I get in the water."
Franklin set the world record to win 200 backstroke gold at the London Olympics last year and also claimed the world title in Shanghai two years ago.
(Reporting by Iain Rogers, editing by Pritha Sarkar and Alison Wildey)
