Phelps victory in Friday's 200m individual medley final came in 1 minute 54.23 seconds, beating his own record time set at last month's US trials.
Phelps touched in more than two seconds ahead of Hungary's Laszlo Cseh who won silver - his third of the competition.
|Phelps' gold haul
Athens 2004: six gold medals
400m individual medley
4x100m free relay
4x200m free relay
200m individual medley
Phelps' US teammate Ryan Lochte took bronze, swimming just 29 minuets after he set another world record winning the men's 200m backstroke.
After securing himself a place in Saturday's 100m butterfly final, if all goes well in that race Phelps should be lined up to break Spitz's record in the 4x100 medley relay final, set for Sunday morning in Beijing.
"The next two races are pretty important," he told reporters in the Water Cube. "I have to conserve as much physical and emotional energy as I can now that I'm down to the last two races."
On a day that two Olympians were thrown out of the competition after failing dope tests, Phelps insisted that his medal sweep had been achieved fairly.
"Anyone can say whatever they want, I know, for me, I am clean," he said.
Phelps' achievements in the pool has undoubtedly been the story of the games so far, but in a sceptical era his record-breaking swims have inevitably drawn scrutiny.
"Anyone can say whatever they want, I know, for me, I am clean"
A series of high-profile doping scandals, involving athletes such as track and field world champion Marion Jones, 100m world record-setter Justin Gatlin and Tour de France winner Floyd Landis, has clouded the sporting world in recent months.
Phelps is among the US Olympians taking part in a special US Anti-Doping Agency initiative known as Project Believe, in which the competitors undergo extensive blood and urine tests beyond the standard testing regime aimed at restoring credibility to sport.
"I did Project Believe, where I purposely wanted to do more tests to prove it," Phelps said.
"People can question all they want, but I have the proof and the facts are the facts."