The European Court of Human Rights on Tuesday condemned Russia for "multiple violations" against Sergei Magnitsky, an anti-fraud crusader who died in custody in 2009 while awaiting trial.
The court determined he had been ill-treated and denied medical assistance in jail, following his arrest over alleged tax evasion.
The case cast a harsh international spotlight on Russian authorities' persecution of whistle-blowers and political opponents, prompting the United States and other countries to pass "Magnitsky Acts" targeting Russians accused of rights abuses.
Magnitsky, an accountant, went public with details of an alleged $230 million fraud by Russian state officials before being arrested in November 2008.
He was charged with two counts of tax evasion committed with William (Bill) Browder, a British financier who ran the Hermitage Capital hedge fund.
Despite repeated demands for medical help for an illness while in custody, Magnitsky was held for months in overcrowded facilities unequipped to provide the necessary treatment, Europe's top rights court found.
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The court's seven judges for the case -- including a Russian -- ruled unanimously that the authorities "deprived Mr Magnitsky of important medical care and had failed to comply with their duty to protect his life".
They also said Magnitsky had been ill-treated by guards and been unjustly held for too long in pre-trial detention.
The judges also denounced an "inherently unfair" posthumous trial that found the accountant guilty.
They ordered Russia to pay 34,000 euros ($37,800) to Magnitsky's wife and mother.
The ruling comes after the US State Department moved in May to impose sanctions on two Russian investigators for allegedly concealing facts over the Magnitsky's detention and death.
The Magnitsky Act allows officials to block the US assets of blacklisted people and bar them from travelling to the United States.
Browder, Magnitsky's former employer, has spearheaded a campaign for similar acts in other countries, with success so far in Britain, Canada and three Baltic states.