Yuriy Hrabovsky's body was found in a forest 125km (78 miles) south of Kiev.
He represented Alexander Alexandrov, who was captured in eastern Ukraine with Yevgeny Yerofeyev last May.
They are accused waging war against Ukrainian troops.
Russia says they were volunteers who had left active service.
Ukrainian President Petro Poroshenko said early this week (in Ukrainian) he was ready to exchange the two Russians for Ukrainian pilot Nadiya Savchenko.
She was sentenced to 22 years in jail after being convicted of directing artillery fire which killed two Russian journalists in eastern Ukraine in June 2014.
The trial of the two Russians in Kiev was stopped at the beginning of the month, after Yuriy Hrabovsky went missing.
Anatoliy Matios says Hrabovsky's body was found during a police excavation of land on a former farming collective.
"Provisionally, I can say that... (he) was killed in a violent way and finished off with a firearm," Mr Matios said.
He added that the lawyer had been robbed and also had an explosive device attached to his leg.
Two men have been detained on suspicion of involvement, but no motive has been mentioned.
Russia's foreign ministry blamed Ukrainian authorities for failing to protect Hrabovsky.
In a statement, it said the lawyer had become a victim of anti-Russian sentiment in Ukraine.
The Russian pair are alleged to be officers in Russia's GRU foreign military intelligence when they were captured during fighting in east Ukraine.
Ukrainian officials say Alexandrov and Yerofeyev confessed to serving in the Russian special services, but later retracted their confessions.
The two men deny charges of terrorism.
Prosecutors have called for life sentences.
Their detention was a major success for Ukrainian forces, which had long maintained that Russian special forces were playing a key role in the military campaign by pro-Russian rebels.
Ukraine's security chief told the BBC at the time that they had been part of a 220-strong GRU team deployed in Luhansk.
While Ukraine and the West have said there is clear evidence of Russian military help, the government in Moscow has argued that only volunteers have joined the separatists in the east.
Source: BBC News