Tymoshenko was sentenced in 2011 to seven years in prison after being convicted on charges she abused her authority when, as prime minister in 2009, she helped broker a natural gas deal with Russia's Gazprom.
Tymoshenko helped lead the Orange Revolution in Ukraine in 2005 and is one of the main leaders of the opposition against Ukrainian President Viktor Yanukovych. Her supporters say charges against her were politically motivated.
The European Court of Human Rights ruled Thursday that some of Tymoshenko's rights were violated by Ukrainian authorities.
The court said, "Tymoshenko's pre-trial detention had been arbitrary; that the lawfulness of her detention had not been properly reviewed; and, that she had no possibility to seek compensation for her unlawful deprivation of liberty."
The court added that Tymoshenko received better care than other inmates despite her health complaints.
The European Union said matters of selective justice and democratic progress must be addressed by Ukraine if it wants to move closer to the bloc. Yanukovych in early April issued pardons for some of Tymoshenko's supporters jailed on corruption charges.
The verdict Tuesday doesn't overturn Tymoshenko's prison sentence. Parties have three months to file an appeal.