Duplantis was born into an athletic family; his American father, Greg Duplantis, who is of Cajun descent, is a former pole vaulter with a personal best of , while his Swedish mother Helena (née Hedlund) is a former heptathlete and volleyball player. His two older brothers, Andreas and Antoine, and his younger sister, Johanna, also took up sports; Andreas represented Sweden as a pole vaulter at the 2009 World Youth Championships and 2012 World Junior Championships, while Antoine dropped pole vault for baseball in high school before heading to Louisiana State University where he became the team's career hits leader in 2019.