20-year-old McKeown swam a blistering final 50m to touch in 2min 04.68sec ahead of Canada's Kylie Masse (2:05.42), whom she also edged into silver over the 100m showdown.
Australian swimming sensation Kaylee McKeown completed an Olympic backstroke double on Saturday, adding the 200m title to her 100m crown.
The 20-year-old swam a blistering final 50m to touch in 2min 04.68sec ahead of Canada's Kylie Masse (2:05.42), whom she also edged into silver over the 100m showdown.
Seasoned Australian campaigner Emily Seebohm (2:06.17) took bronze in her fourth Olympics.
McKeown paced her race perfectly, narrowly behind Masse until 150m when she put her foot to the floor to comfortably win gold.
"I always try to backend my 200 as much as I can," said McKeown, who last year lost her father Sholto to brain cancer.
"I really just tried to put on my race plan, which is take the first 100 out as easy as possible and then just really give it a crack in that last 100."
McKeown, the 100m world record holder, was always favourite, almost two seconds faster than any other swimmer in the field heading into the Games, recently clocking the third-quickest time in history.
Masse said she was delighted to get on the podium again.
"It's fun. It's definitely more painful that the 100," she said. "But we all know it's going to be painful, it's going to be a stinger, you just have to stick it out and have faith in your training."
For veteran Seebohm, it capped a stellar career that has brought her six Olympic medals over four Games -- two gold, three silver and a bronze.
"I was trying to not get distracted by the girls next to me. I had my race planned and it worked perfectly," she said.
McKeown payed tribute to her 29-year-old team-mate.
"I had a bit of a tear in my eye, to be honest," she said. "Em has been around for so long and I speak so highly of her for a reason. It made sense for her to be up there with me."