Phelps has already triumphed in the 400m individual medley and 4x100m freestyle relay and looks to be improving with each race.
The 200m freestyle has a medal which eluded him in Athens when he was a bronze medallist behind Australian Ian Thorpe.
"Phelps swam so fast," said silver medallist Park. "It is my honour to compete with him."
Phelps's ninth career Olympic gold draws him level with Mark Spitz Carl Lewis, Paavo Nurmi and Larysa Latynina and keeps him on course to beat Spitz's 36-year-old record of seven golds in a single Games.
Racing out of lane six, Phelps quickly surged into the lead and led by a full body length halfway through the second of four lengths.
Phelps will race for his fourth medal on Wednesday in the 200m butterfly, yet another event in which he holds the world record.
He advanced to the final as the fastest qualifier just moments after receiving his 200m freestyle gold medal.
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