Phelps finished the final in 1 minute 54.66 seconds on Thursday (August 11), almost two seconds ahead of Japan's Kosuke Hagino at the Olympic Aquatics Stadium. China's Wang Shun took bronze in 1:57.05, reports Xinhua. (Phelps win 2 golds to take tally to 21)
While the landmark is a swimming first, two other athletes have achieved the feat in other Olympic sports: Al Oerter in discuss and Carl Lewis in long jump.
Phelps could also perform the so-called four-peat in the 100m butterfly here.
Thursday's victory was Phelps' 22nd Olympic gold medal as he continues to extend his record as the most successful Olympian of all time.
The 31-year-old has now won each of his four finals in Rio so far: the 200m butterfly, the 4x100m freestyle, the 4x200m freestyle and the 200m individual medley.
On Friday, he will compete in the final of the 100m butterfly and he is expected to also take part in the 4x100m medley relay.