Heyns, who already holds world records at 50 and 100m, set the record with a swim of 2min 24.69sec in the preliminaries, breaking the mark of 2:24.76 set by Australia's Rebecca Brown in 1994. In the final on Saturday, Heyns lowered the mark to 2:24.51.
Heyns, who won gold medals in the 100 and 200m breaststroke at the 1996 Olympics, said: "Every time I swim well like that, it blows me away. It's like I'm blessed."
America's Tom Wilkens won the 200m breaststroke in 2:16.09 and the 400m individual medley in 4:23.79.
Joanne Malar also won twice, taking the 200m freestyle in 2:02.22 and the 400m individual medley in 4:42.63.
South Africa's Ryk Neethling won his third event of the meet, taking the 200m freestyle in 1:50.71.
The Olympic gold medalist Jenny Thompson won her second event, the 50m freestyle, in a meet-record 25.75. Roland Schoeman won the men's 50 in 22.93.
The meet, named for the American woman who set and still holds records for 400, 800 and 1500m freestyle a decade ago has attracted more that 700 competitors.
Success for Adam Faulkner and David Bennett brought Britain's medal tally to a record 12 at the European Junior Championships in Moscow on Saturday.
Faulkner, from the Nova Centurion club in Nottingham, grabbed silver in the 1500m freestyle to add to his 400m bronze.
Bennett, of Beckenham, who had already earned bronze in the 100m butterfly, won silver in the 50m butterfly.Reuse content