The child was in the locker room to change after swimming when the naked man touched him in a sexual manner, police said.
The boy told his parents that night. He was not injured.
According to Fairfax Parks and Recreation, "In locker rooms, children under 10 must be accompanied by a family member over 16." Children 7 or older must be in gender-appropriate locker rooms, but family changing rooms are available for mothers and sons and fathers and daughters.
The man is about 6 feet tall, slender, in his late 30s with a tan or olive complexion, according to police.
Anyone with information about the case should call police at 703-691-2131 or Crime Solvers at 866-411-TIPS/8477 or visit Crime Solvers online.