Phelps ended an historic day at the London pool by anchoring an 800-meter freestyle relay victory for the United States, one that brought him the 19th medal over the course of three Olympics.
That is one more than the 18 medals won by gymnast Larysa Latynina. Her record had stood for 48 years and the one the 27-year-old Phelps now owns should last at least that long.
Phelps was clearly expected to break the record during his stay in London since he needed only three medals to do so and he carries with him the reputation of being the greatest swimmer in history.
The road to the record turned out to be bumpier than expected, however, and included a bittersweet addition to his medal collection early in Tuesday's program.
Phelps, already the holder of the Olympic record for most gold medals, looked to have won his 15th when he built a seemingly comfortable lead in the 200-meter butterfly -- his favorite race.
But that lead dwindled and as Phelps came to the conclusion of the event, he made a rookie swimming mistake. Caught between strokes, he coasted to the wall instead of making a final half lunge to get there as quickly as possible.
While Phelps was coasting, Chad le Clos of South Africa was lunging and le Clos beat Phelps by 0.05 of a second.
It was still a medal and it was the one that tied the record, but the fact that it was not a gold one a huge low for Phelps.
He went through the medal ceremony, had his picture taken alongside le Clos and did all the things required of him.
The next time he jumped in the water he had a lead of almost 4 seconds, given to him by Ryan Lochte, Conor Dwyer and Ricky Berens over the first three-quarters of the relay.
France's Yannick Agnel trimmed a second off that lead, but there was never any doubt Phelps would set his career record on a gold-medal note.
It was the second gold medal of the night for the Americans in the pool, the other one won by Allison Schmitt in an impressive performance in the 200-meter freestyle. China's Ye Shiwen also put together a blistering final 50 meters of freestyle swimming to win the women's 200-meter individual medley.
American Caitlin Leverenz finished third in the individual medley, giving the United States four medals in swimming for the fourth straight day.
In only one swimming event contested thus far at the London Games has the United States failed to win a medal.
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