Dart told the Chicago Sun-Times 19-year-old William George Bundy was among at least 33 people Gacy killed between 1972 and 1978.
Illinois executed Gacy in 1994.
Dart's office began an effort in October to use DNA technology to put names to eight unidentified victims whose bodies were exhumed earlier this year.
Bundy's sister Laura O'Leary and brother Robert Bundy submitted DNA samples that allowed the University of North Texas to link one of the unidentified bodies to William Bundy.
O'Leary said her family will hold a memorial service for her brother next year.
"Today is a terribly sad day for my family," she said. "But it is also a day that provides closure."