Phelps broke Soviet gymnast Larisa Latynina's career record of 18 medals; she won nine golds, five silvers and four bronzes from 1956-64.
Earlier in the day, Phelps won the silver medal in the 200-meter butterfly after making a shocking blunder at the end of the race. He led the entire way during the event and tried to glide to the wall after his final stroke. He flipped away his cap in disgust when he realized he was out-touched at the wall by South Africa's Chad le Clos.
Le Clos won in a time of 1 minute, 52.96 seconds, while Phelps finished in 1:53.01. Japan's Takeshi Matsuda took the bronze in 1:53.21.
U.S.' Allison Schmitt set an Olympic record when she won the 200-meter freestyle, clocking 1 minute, 53.61 seconds. U.S. teen Missy Franklin, who won gold in the 100-meter backstroke Monday, finished fourth, missing bronze by .01.
Also on Tuesday, America claimed its first Olympic gold medal in women's gymnastics since 1996. The winning score of 183.596 was a whopping five points ahead of Russia. Romania won the bronze medal. This year's women's team has been hailed as the strongest the U.S. has ever had.
In men's volleyball, the U.S. team swepts Germany in preliminary pool play. Clay Stanley scored 16 points, while Matt Anderson added 15 points.
In women's soccer, the U.S. clinched first place in its group with a decisive goal by Abby Wambach to defeat North Korea 1-0.
In round two of men's tennis, Andy Roddick left the court with his head bowed after being knocked out by Noval Djokovic 6-2, 6-1. The second-ranked Serb had 34 winners on Centre Court at the All England Club. Roddick had 12. Roddick is a three-time Wimbledon finalist.
Venus Williams advanced to the third round of women's tennis, beating Canadian Aleksandra Wozniak 6-1, 6-3. On Monday she beat Italy's Sara Errani and got herself one step closer to winning a record fourth gold medal. Williams will go on to play 7th seeded Angelique Kerber of Germany.
Younger sister Serena Williams won't play mixed doubles at the Olympics, and the two U.S. teams will instead feature four doubles specialists -- the Bryan brothers and Liezel Huber and Lisa Raymond.
Raymond and Mike Bryan will be reunited after winning their third Grand Slam title together at Wimbledon this summer. The other U.S. entry will be Huber and Bob Bryan, who have won two major titles together.
In the equestrian event, Queen Elizabeth's granddaughter Zara Phillips helped team Britain take silver. Germany won gold and New Zealand won bronze. Princes William and Harry and William's wife, Kate, were in the stands to cheer on Phillips and the rest of the British squad. But Phillips and teammate Nicola Wilson knocked down rails, dashing Britain's chances for gold.
Team USA's Vincent Hancock took the gold medal in the men's skeet competition. The win makes for his second straight gold in shooting. Hancock also topped his own Olympic record by hitting 123 targets in qualifying earlier in the day. He was a perfect 25-for-25 in the medal round to wrap up the gold.
Hancock's win also gives the U.S. a sweep in the sport. El Monte native Kimberly Rhode won the women's competition earlier this week.
Hancock clinched the gold on his next-to-last shot, and missed only twice in 150 tries.
Anders Golding of Denmark won silver in the competition, and Qatar's Nasser Al-Attiya won the bronze after a shoot-off with Russia's Valery Shomin.
The Associated Press contributed to this report.