Dunja Mijatovic said her office had documented hundreds of cases of violence against the media, including murder, assault and kidnappings, since
anti-government protests began in Kiev late last year.
"I am alarmed by the rapidly deteriorating conditions and climate for the media," she said.
"The manipulation of the media and the information war we are experiencing has to stop. Failure to do so will fuel the conflict and contribute to an escalation of the crisis," she said in a statement released by the Organisation for Security and Co-operation in Europe.
The Vienna-based body documented over 300 cases of violence against members of the media, including murder, physical assaults, and the destruction of equipment since November.
Several television stations have already been illegally switched off, with broadcasts in Crimea and the restive east of Ukraine replaced with broadcasts from Russia, the report said.
With key presidential elections taking place in Ukraine this weekend, the OSCE said journalists "must be able to do their reporting in a free and safe manner", and called on law enforcement agencies "to offer all possible support".
Sunday's vote is seen as the most crucial since Ukraine's independence in 1991 with the country facing the threat of partition and teetering on the brink of economic collapse.
"If actions are not urgently taken, the consequences will be too grave to even imagine," Mijatovic said.
"The on-going attacks on journalists are nothing short of gross and severe violations of fundamental human rights.
"Journalists are deliberately targeted for doing their job, trying to tell the outside world of the events… taking place in Ukraine."
OSCE staff have been targeted in the conflict, with seven observers taken hostage in April in the restive eastern Ukrainian town of Slavyansk. They were released earlier this month.