In ruling the law unconstitutional, Miami-Dade Circuit Judge Milton Hirsch said that the changes should have been crafted by the Florida Supreme Court instead of by the Legislature.
The Miami Herald reports that the 14-page order is a victory for prosecutors who have firmly opposed the law. Critics have said the law makes it easier for defendants to get away with murder and other violent crimes.
"As a matter of constitutional separation of powers, that procedure cannot be legislatively modified," Hirsch wrote in the order, The Herald reports.
The Legislature modified the 2005 statute and Gov. Rick Scott signed it into law in June. The bill was backed by the National Rifle Association.
CBS Miami reportsMonday's ruling is considered a victory for prosecutors who believed the changes in the law would make it easier for defendants to get away with murder.
The updated law required prosecutors to shoulder the burden of disproving a self-defense claim — making them prove by "clear and convincing" evidence that someone wasn't acting in self-defense. State attorneys have said that essentially forces them to unfairly try a case twice, making it easier for criminals to escape justice.
There was also concern that the update made it easier for judges to dismiss criminal charges if they deemed the act was self-defense, without a jury hearing the case.
The Herald reports that in Miami-Dade County, judges have tossed out several high-profile murder cases after pre-trial immunity hearings. But they've also allowed others to go to a jury.
Florida's "Stand Your Ground" law has long been criticized for fostering a shoot-first mentality, by eliminating a citizen's duty to retreat before using deadly forcein responding to an apparent threat.
The judge's ruling likely will lead to legal wrangling in the appellate courts and the Florida Supreme Court. Kylie Mason, a spokeswoman for Attorney General Pam Bondi, told The Associated Press they would appeal the order.
Scott spokesman John Tupps also said the governor's office is reviewing the judge's ruling.
The 2012 killing in Florida of unarmed teenager Trayvon Martin by neighborhood watch volunteer George Zimmerman opened a debate about the limits of self-defense, and it hasn't let up since. Zimmerman was acquitted of second-degree murder after jurors received instructions on Florida's "Stand Your Ground" law.