Peter Jeffrey Marshall (born March 9, 1982) is an American competition swimmer who specializes in the backstroke.

He earned his first national championship in the 100-meter backstroke in the summer of 2001 in Clovis, California.

While at Stanford University, he won three consecutive Individual NCAA Titles in the 100-meter backstroke.

In 2004 he was named Pac-10 Swimmer of the Year for his first individual world record set at the NCAA Championships in Long Island, New York.

His cousin, Gary Marshall, also swam for Stanford after transferring from the University of Virginia.