Friday's decision by Nebraska's Surpreme Court came close on the heels of a decision by the San Fransisco appeals court to throw out more than 100 death sentences in three western US states.
In both instances, the courts were acting on a 2002 US Supreme Court case in which the justices ruled that death sentences imposed by judges, and no juries, are unconstitutional.
The San Fransisco appeals court and the Nebraska Supreme Court applied the ruling retroactively.
In its decision, the Nebraska Surpreme Court said although there was enough evidence to prove that Raymod Mata Jr. had killed Adam Gomez, it had no choice but to grant him a new sentencing hearing in view of the 2002 Supreme Court ruling.
Nebraska attorney general Jon Bruning said he was confident that Mata would be resentenced to death.
Mata had been convicted of killing the child in a fit of jealously in 1999.
He was involved with the child's mother, but grew upset because he believed the child had rekindled a relationship between the mother and his biological father.
On killing the child, Mata had dismembered his body and disposed of some remains by flushing them down the sewer and feeding others to a dog.