Leonidas, a runner who competed between 164 and 152 BC according to Olympic historians, is the only other athlete to have won 12 solo titles.
There is plenty of documentary evidence to attest to the dominance of Phelps, winner of a record 21 golds and 25 medals in an astonishing career spanning five Games that is far from finished.
The achievements of Leonidas rely on somewhat sketchier accounts, provided much later by Philostratus the Athenian and Pausanias who hailed the runner's 12 victories through four Olympiads.
The sentiments expressed by Phelps in the early hours of Wednesday after winning the 200m butterfly title would surely have resonated with Leonidas, even if the Greek had to compete with a shield and helmet.
"That's a lot of medals," grinned the American. "We got a lot of medals. It's just insane. It's mind-blowing."
Phelps has by far the most of any Olympic athlete and is the only one to win eight golds at a single Games, an achievement that earned much fanfare plus a $1 million bonus in Beijing in 2008.
In Rio, he has so far taken three golds - one individual and two relays - and on Thursday he could become the first swimmer to win the same event, the 200m individual medley, at four successive Games.
Tuesday's 200m butterfly gold made him the oldest swimmer to win an individual Olympic title at 31 years and 40 days.
Put into perspective, if Phelps were a sovereign state he would rank 52nd in the all-time lists in terms of total medals won. On golds alone, he would be in the top 40.
India, a nation of a billion people, has competed at every Olympics since 1920 and only ever won nine golds, with not even a medal in swimming.
Michael Phelps Olympic history
Michael Phelps Olympic history
Team USA swimmer Michael Phelps prepares to swim laps during a practice session at Northside Swim Center in San Antonio, Texas, July 16, 2016. Mandatory Credit: Soobum Im-USA TODAY Sports/File Photo
Michael Phelps (C) of the U.S. swims the butterfly stroke during his team's victory in the men's 4x100 meters medley relay at the National Aquatics Center during the Beijing 2008 Olympics August 17, 2008. Picture taken with underwater camera. REUTERS/Wolfgang Rattay/File photo
Michael Phelps of the U.S. poses with his gold medal after winning the men's 4x100m medley relay final during the London 2012 Olympic Games at the Aquatics Centre August 4, 2012. REUTERS/Jorge Silva/File photo
Michael Phelps and Garrett Weber-Gale (L) of the U.S. celebrate after winning the men's 4x100m freestyle relay swimming final at the National Aquatics Center during the Beijing 2008 Olympic Games August 11, 2008. REUTERS/David Gray (CHINA)
Gold medallist Michael Phelps of the U.S. listens to his national anthem during the men's 200m individual medley victory ceremony during the London 2012 Olympic Games at the Aquatics Centre August 2, 2012. REUTERS/David Gray
Members of the men's 4 x 200m freestyle relay team, Michael Phelps, Conor Dwyer, Ryan Lochte and Ricky Berens (L-R) hold their national flag as they celebrate their gold medal win in the men's 4x200m freestyle relay victory ceremony during the London 2012 Olympic Games at the Aquatics Centre July 31, 2012. Phelps won the record 19th Olympic medal on Tuesday when he joined forces with his U.S. team mates to win the 4x200m freestyle relay. REUTERS/Jorge Silva
Michael Phelps of the U.S. competes in the men's 400m individual medley swimming final at the National Aquatics Center during the Beijing 2008 Olympic Games August 10, 2008. Picture taken with underwater camera. REUTERS/Wolfgang Rattay (CHINA)
Michael Phelps of the U.S. holds up his award recognising him as the most decorated Olympian, during the London 2012 Olympic Games at the Aquatics Centre August 4, 2012. Phelps ended his incredible Olympic career on the perfect note on Saturday, winning his 18th gold medal for the United States in the men's medley relay, the last time he will swim a competitive race. REUTERS/Brian Snyder
A combination photo shows Michael Phelps of the U.S. holding each of his eight gold medals in the swimming competition at the Beijing 2008 Olympic Games. Phelps won his eighth gold medal on Sunday, beating Mark Spitz's record of seven gold medals won at a single Games in 1972. The medals were won in: (Top row L-R) 400m individual medley (August 10), 4x100m freestyle relay (August 11), 200m freestyle (August 12), 200m butterfly (August 13). (Bottom row L-R) 4x200m freestyle relay (August 13), 200m individual medley (August 15), 100m butterfly (August 16), and 4x100m medley relay (August 17). REUTERS/Staff (CHINA)
Michael Phelps of the U.S. looks down at the pool after his team was beaten to the gold by France in the men's 4x100m freestyle relay final during the London 2012 Olympic Games at the Aquatics Centre July 29, 2012. REUTERS/David Gray
Photographers surround Michael Phelps of the U.S. after he won his eighth gold medal of the games at the National Aquatics Center during the Beijing 2008 Olympic Games, August 17, 2008. REUTERS/Gary Hershorn (CHINA)
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Mexico and Indonesia, two more of the world's most populous nations, have just 19 golds between them.
Phelps has won more golds in the pool than China's men and women combined, and he has not finished yet.
"He is just proving he is easily the greatest swimmer of all time and I am excited to see what he does next because once that guy gets hot you can't really stop him," said relay team mate Conor Dwyer.