At the 2012 Summer Olympics in London, she again broke her American record and set an Olympic record with a time of 56.25 seconds in her 100-meter butterfly qualifying heat. In the 100-meter butterfly final, she won the gold medal and set a new world record with her time of 55.98. Vollmer also competed in the 4×200-meter freestyle relay. She swam the second leg with a time of 1:56.02, as the U.S. team won gold with a time of 7:42.92. In her final event, the 4×100-meter medley relay, Vollmer won another gold with Missy Franklin, Rebecca Soni and Allison Schmitt. Swimming the butterfly leg, Vollmer recorded a split time of 55.48, as the U.S. team set a new world record with a time of 3:52.05, bettering the previous record of 3:52.19 set by China in 2009.