Readers were allowed to vote via Facebook, Twitter and Instagram, Sports Illustrated said in a release.
About 21 percent of voters cast their votes for Phelps' performance in London, where he won gold in the 100-meter butterfly and 200-meter medley, and was on the winning 400-meter medley and 800-meter freestyle relay teams. He earned silver in the 200-meter butterfly and 400-meter freestyle relay and ended up as the most decorated Olympian of all time, with a record 18 golds and 22 overall medals won in three Olympic games.
Gabby Douglas winning gold at the Olympics for gymnastics came in second on the list of five with 19 percent.
The two Olympic moments were followed by Peyton Manning being traded to the Denver Broncos, Jamaican sprinter Usain Bolt setting records at the Olympics, and the San Francisco Giants winning the World Series in a four-game sweep over the Detroit Tigers.