In December, a video containing intimate and illegally obtained images of the journalist appeared on the Internet.
His Kyiv apartment was also picketed last week by some 50 protesters who carried threatening posters and accused Portnikov of "immoral behavior."
RFE/RL President Kevin Klose called the actions "an orchestrated effort to discredit independent, professional journalists, reminiscent of the most reprehensible tactics of Soviet times."
Portnikov is a prominent Ukrainian journalist who heads the TVi channel.
He began working with RFE/RL's Ukrainian Service in the early 1990s.
He is a member of the public committee of Euromaidan, the organizing body of pro-EU demonstrations that emerged after President Viktor Yanukovych rejected an agreement on closer ties with the European Union in November.