Rahmani told Radio Farda that he was due to travel to Denmark on January 13, to give a speech and receive a prize awarded by the local section of the freedom of expression organization.
But he said authorities prevented him from leaving the country without giving him a reason.
The authorities "didn't mention any specific reason for banning me from leaving the country," he said. "They only implied that the trip would cause trouble for the establishment and for myself. Despite my efforts, the ban on me leaving the country was not removed."
RSF says that Iranian authorities often use "a ban on leaving the country" against independent voices in an attempt to cut them off from the external world.
Rahmani's work as a journalist and also his political activities have brought him into frequent conflict with the establishment. Between 1981 and 2005, he reportedly was sentenced to a total of 5,000 days in prison.