Cherkassk province prosecutors said Vasily Sergiyenko was abducted in his home city of Korsun-Shevchenkivskyi on Friday evening, and later murdered.
The nationalist Svoboda party, of which Mr Sergiyenko was a member, said the reporter was found with stab wounds and signs he had been beaten about the head and knees.
Svoboda said the killing bears the hallmarks of a politically-motivated hit.
The party, whose leader Oleh Tyahnybok is running in the May 25 presidential election, said Mr Sergiyenko and other Svoboda representatives had received threats over the past week.
Ukraine has threatened to take Russia to an arbitration court if talks with Moscow fail to roll back hikes in the price of natural gas. Last week Russia nearly doubled the price Ukraine pays for its gas.