The justices said that the first jury charged with determining Daryl Atkins' mental state should not have been told that he had earlier been sentenced to death for a 1996 murder because the information was prejudicial.
The jury finding became necessary after the U.S. Supreme Court ruled in Atkins' case in 2002 that executing the retarded is unconstitutional.
The court left it to the states to define retardation and sent Atkins' case back to circuit court. The jury determined last August that Atkins was not retarded, allowing his death sentence to stand.
In the appeal to the Virginia Supreme Court, Atkins' lawyers argued that this jury should not have been told about the conviction and sentence because its sole duty was to determine whether Atkins is mentally retarded. Lawyers for the state argued that the jury was entitled to all the information, but the high court disagreed.
"The fact that the jury knew a prior jury had sentenced Atkins to death prejudiced his right to a fair trial on the issue of his mental retardation," Justice Cynthia D. Kinser wrote.
Atkins was convicted of the 1996 robbery and shooting death of Air Force enlisted man Eric Nesbitt.