Phelps, the record holder for most Olympic medals with 20, including an incredible collection of 16 golds, has announced his decision to quit.
By winning his 19th Olympic medal, he broke Soviet Union gymnast Larisa Latynina's record haul of 18. Latynina's mark had stood for 48 years.
The American will swim the 100-metre butterfly on Friday. This will be his last individual event. However, he will swim in the team event (4x100 metres medley final) on Saturday, which will be his final event at the Games. On Thursday, he won his 20th medal by capturing gold in 200m individual medley. This was his first yellow metal in London.
"No more competitive swimming, no more club meets. I will definitely retire after London Olympics," Phelps was quoted as saying by China's Xinhua news agency on Thursday.
Upon winning 200m IM, he became the first swimmer in Olympic history to win the same event in three successive Games.
"To be able to win the gold medal and repeat three times is something pretty special. I'm pretty pleased with gold," the 29-year-old said.
Phelps is participating in his fourth and final Olympics.
There has already been debates whether he is the greatest Olympian of all time.
"I think you can say it is self-evident that he is the most successful. I am not sure he is the greatest. It is a pretty good haul, but who is the greatest? In my opinion he is probably not," stated Sebastian Coe, the chief of London Olympics 2012, was quoted as saying in the media. Coe is a double 1,500-metres Olympic champion.