The action, taken by the Baltimore state's attorney in a pretrial hearing in the case of officer Garrett Miller, comes after the acquittal of three of six Baltimore police officers charged in the case.
Gray, 25, suffered a severed spine while being transported in a police van following his arrest on April 12, 2015. He died a week later.
The case set off riots in Baltimore and added fuel to a simmering national debate over the treatment of African Americans by police.
Baltimore state's prosecutor Marilyn Mosby brought charges ranging from second degree murder to assault, manslaughter and reckless endangerment against the officers involved, but has been unable to win a conviction in any case that has come to trial.
Three officers were found not guilty and a fourth case ended in a mistrial.
Miller was the next to face trial when the charges were dropped.
"At the motion's hearing for State v Garrett Miller this morning, the Baltimore City State's Attorney’s Office decided not to prosecute the remaining cases related to the arrest and death of Freddie Gray, including State v White and State v Porter," a spokesman for the Maryland judiciary said.
A spokeswoman for the Baltimore state's attorney also confirmed that all charges have been dropped.
