Margaret Josephine Hoelzer (born March 30, 1983) is an American former competition swimmer, Olympic medalist, and former world record-holder.

In 2007, Hoelzer moved to Charlotte, NC to train with Coach David Marsh and was sponsored by Speedo.

Her grandfather Helmut Hoelzer invented the first fully electronic analog computer and was a member of the Wernher von Braun Operation Paperclip team.

According to SwimSwam magazine, she is considered one of the top 10 women swimmers who did not win an Olympic gold medal.

In the 2004 Summer Olympics in Athens, Greece, she placed 5th in the 200-meter backstroke, with a time of 2:10.70.

On July 5, 2008, Hoelzer broke her former Auburn University roommate Kirsty Coventry's world record of 2:06.39 in the 200-meter backstroke, with a time of 2:06.09, ultimately qualifying for her second event in the 2008 Olympics.