The 8-1 ruling came in a challenge filed by death row inmate Timothy Lee Hurst, 37, convicted of murdering restaurant co-worker Cynthia Harrison, who was bound, gagged, and stabbed more than 60 times in 1998.
A jury recommended a death sentence for Hurst, but it was the judge who held a separate hearing and determined there were sufficient aggravating circumstances to impose the death penalty, the high court said.
"We hold this sentencing scheme unconstitutional," the ruling said. "The Sixth Amendment requires a jury, not a judge, to find each fact necessary to impose a sentence of death. A jury's recommendation is not enough."
The order sent Hurst's case to the Florida Supreme Court for further proceedings "not inconsistent with this opinion," the ruling said.
The decision could open the door to other sentencing challenges from many of Florida's 385 men and five women now on death row,
who together represent more 10% of the nation's approximate 3,000 convicts awaiting execution, according to the American Civil Liberties Union and legal experts.
Also, the ruling could lead to other legal challenges around the nation, specifically in Alabama and Delaware where death sentencing procedures also delegate some decisions to judges instead of juries, said Cassandra Stubbs, director of the ACLU Capital Punishment Project.
"This shows that the Supreme Court continues to apply close scrutiny to the death penalty. I think it's really a harbinger of the day coming when the Supreme Court is going to strike it down," Stubbs said.
Legal analyst Mark McBride, a criminal defense attorney in California, said the Florida Supreme Court "will order -- and quite soon -- that prisoners who had been sentenced under the current, albeit flawed, scheme will have their sentences commuted to life without the possibility of parole."
In turn, the U.S. Supreme Court "will now be occupying itself much more in earnest with states who have death penalty schemes which seem peculiar or archaic or poorly thought out," McBride said.
Justice Samuel Alito was the sole dissenter in Tuesday's ruling and disagreed that the Sixth Amendment requires juries to make specific findings authorizing a death sentence.
Alito noted how Harrison was savagely murdered, and he added her death could have taken as long as 15 minutes.
"More than 17 years have passed since Cynthia Harrison was brutally murdered. In the interest of bringing this protracted litigation to a close, I would rule on the issue of harmless error and would affirm the decision of the Florida Supreme Court," Alito wrote.