The list includes journalists from Agence France-Presse and other global media organisations as well as Ukrainian and Russian outlets.
The hackers said on their website they were making the list public "because these reporters cooperate with the rebels of a terrorist organisation".
Ukraine identifies the insurgents as "terrorists" who receive direct backing from the Russian armed forces -- a charge Moscow denies.
Reporters who entered the war zone needed to receive special permits from the separatist authorities in order to work in the territory they controlled since the start of the conflict in April 2014.
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"After being charged with 'working for terrorists' and having their personal data, telephone numbers and emails disclosed, these reporters began receiving threatening phonecalls and letters," said the letter published on the hromadske.Tv news site.
"And some Ukrainian politicians have already called on branding these reporters as 'enemies of Ukraine' and shutting down their ability to work."
The letter was signed by 24 reporters as well as the RSF representative.
"There will be no sanctions applied against reporters on this list. They did not break any laws," Tetyana Popova told AFP.
She added that the website on which the data was leaked was not associated with the Ukrainian authorities.
The eastern revolt has claimed the lives of more than 9,300 people and driven more than 1.5 million from their homes.