Rosario DiGirolamo pleaded guilty to aggravated manslaughter in Trenton for the killing of Amy Giordano, 27, in 2007.
DiGirolamo is expected to be sentenced to 25 years at his sentencing in February.
Jury selection was set to begin on Tuesday.
DiGirolamo, 36, admitted he hit Giordano over the head with a hammer after she attacked him with a crowbar during an argument.
Prosecutors had previously said that DiGirolamo cut up Giordano's body and dumped body parts in a pond in Staten Island, N.Y.
He then drove to Delaware, where he abandoned the couple's 11-month-old son in a parking lot at Christiana Hospital. A nurse found the boy unharmed.