Activist attorney Iryna Nozdrovska spent two years working on the case against Dmytro Rossoshansky -- a Kiev judge's nephew convicted of driving under the influence in a fatal car crash in September 2015.
Rossoshansky was jailed in June 2017 but immediately filed an appeal.
The high-profile case was seen a test of the Ukrainian justice system's ability to fairly prosecute people with links to the upper echelons of power who had seemed untouchable prior to the pro-EU revolution that swept Kiev in 2014.
"She had succeeded in demonstrating to the court that there was plenty of evidence that Rossoshansky had been under the influence of drugs when he caused the accident," the Kharkiv Human Rights Protection Group said in a statement.
Rossoshansky was sentenced in June to seven years behind bars.
The Kharkiv Human Rights Protection Group said Nozdrovska had received constant threats during the trial "from Rossoshansky himself, and from his mates".
A Kiev court turned down Rossoshansky's appeal last Wednesday and ordered him to remain in a detention centre for another 60 days while the case underwent further hearings.
Kiev police said the 38-year-old mother of one was reported missing on Friday and that her body was discovered Monday.
"Iryna Nozdrovska's body, reportedly naked, was found in a river in the Vyshhorod district near Kiev," the rights group said.
Parliament member Mustafa Nayyem -- a prominent leader of the 2014 street protests that pulled Ukraine out of Russia's orbit -- wrote on Facebook that Rossoshansky's father "warned Irina during (Friday's) hearing: This won't end well for you".
The Kiev region's police chief Dmytro Tsenov denied having received any reports of intimidation or other complains from Nozdrovska.
But more than 100 people rallied outside the Kiev police headquarters shouting "shame" and demanding an impartial investigation into Nozdrovska's death.
Foreign Minister Pavlo Klimkin underscored the national significance of the incident by calling it "a challenge to the state".
This is "a test of our society's ability to protect female activists and to ensure justice as a whole".
US 'shock'
The former Soviet republic has come under growing criticism from its Western allies for failing to implement the institutional changes promised by the new brand of leaders who replaced the Russian-backed leadership nearly four years ago.
Diplomats and economists both identify Ukraine's corrupt court system as one of the biggest impediments to foreign investment and public trust in the authorities.
The US embassy issued a blunt statement Tuesday saying it was "shocked and saddened" by the activist lawyer's death.
"Those responsible must be brought to #justice," the US embassy said on its official Twitter account.
The Kiev region's police said it had already interrogated 50 people and suspected that Nozdrovska's murder was linked to her work on Rossoshansky case.
"We have ascertained that this murder happened because of the last court case in which she played such a big role," the Interfax-Ukraine news agency quoted police spokesman Mykola Zhukovych as saying.
But Zhukovych added that he could not yet rule other motives.
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