Efimova, 25, touched the wall at two minutes, 19.64 seconds after a powerful final 50 metres with Bethany Galat of the United States winning silver at 2.13 back and China's Shi Jinglin taking bronze at 2.29.
After winning bronze in the 100m, Efimova, who served a 16-month ban for doping up until February 2015, now has five world golds spanning four championships.
America's Lilly King, 20, the 100m Olympic and world champion and Efimova's main rival over the shorter distance, led after the first 50 metres, but faded to finish fourth at 2.47 back.
"I expected to win this event, it wasn't a surprise," said Efimova, who avoided the post-final press conference as she is racing again on Saturday.
"Lilly King congratulated me - I didn't see her during the race, as she was in lane eight, but she was on my mind throughout."
Australia's Taylor McKeown, the 2014 Commonwealth champion, was seventh at 3.42secs.
The 21-year-old Galat was delighted with silver, her first major medal.
"I don't know Yuliya personally, her time was incredible - she's a heck of a swimmer and a heck of a breaststroker," said Galat.
"I'm just soaking up every second, coming in representing America is very special.
"We're on fire and it's contagious - that happened so fast it doesn't feel like it happened.
"The pain didn't really matter, it was just exciting.
"I just crushed my nerves and ran my own race, I could hear the crowd it was contagious - I definitely used that energy."
Shi, who also finished third in the Olympic final last year, was happy with her bronze.
"I am happy I could improve significantly from the semi-final, I hoped it would be enough for a medal," said the Chinese.
"As a matter of fact, not much was missing from second place."
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