The presenter, who is 18 years old, was freed at about 2.30pm local time after a police officer persuaded his captor to release him.
The hostage-taker, a 17-year-old former intern, was later arrested after climbing onto the building's roof.
He entered the premises of Radio Latitude in Pont-Sainte-Marie, near Troyes in eastern France, at 10.45am and burst into the studio while a broadcast was taking place, announcing that he had a gun and two magazines of bullets.
He demanded to see the radio's director, Sylvain Jourdain, whom he apparently assaulted in March, an act for which he faces charges.
He said he had a grudge against him over events that took place when he was at a trainee at the radio at the age of 14.
A complaint to police had not been followed up, he said.
Police surrounded the building on Thursday and top law and order officials came to the scene.