U.S. District Court Judge Benson E. Legg said the requirement "impermissibly infringes the right to keep and bear arms" guaranteed by the Second Amendment, The Baltimore Sun reported.
"A citizen may not be required to offer a 'good and substantial reason' why he should be permitted to exercise his rights," Legg wrote in a 23-page opinion. "The right's existence is all the reason he needs."
The ruling, based on a Baltimore County man's lawsuit, drew praise from gun enthusiasts and criticism from those who seek to limit access to firearms.
"Guns don't kill people; people kill people," said Chuck Ammann, a 67-year-old Parkville man who has owned a gun since he was 10.
Ammann said "every day you read in the paper that the bad guys have [guns] and they do not have permits."
But Jonathan Lowy, director of the Brady Center's Legal Action Project, said it's "perfectly reasonable and prudent for Maryland to have law enforcement decide who has a demonstrated need to carry loaded guns in public."
"There's a whole other level of risk that the public is exposed to when civilians carry loaded guns on public parks, streets, in restaurants, what have you."
Lowy called the decision a "very dangerous precedent" that is not supported by case law and said he expects it to be reversed.
The Brady Center submitted a brief in the Maryland federal case.