RIA Novosti reported the investigation focuses on alleged tax evasion in the late 1990s when Tymoshenko headed United Energy Systems of Ukraine, then the main importer of natural gas from Russia.
The charges had been dropped under the statutes of limitations.
Prosecutors had earlier begun investigating United Energy's alleged $405 million debt to the Russian Defense Ministry.
Tymoshenko's lawyer, Serhiy Vlasenko, said tax police notified her Friday that the investigation into four criminal cases against her has resumed.
Tymoshenko, 50, was sentenced last month to seven years in prison after being convicted of abuse of office in awarding gas contracts to Russia in 2009.
The European Union, and the United States have said Tymoshenko's sentence for exceeding her authority was a politically motivated and selective use of the judicial system. Russia has also criticized the sentence as politically motivated.
She has appealed.
Tymoshenko has been heavily prosecuted since a rival, Viktor Yanukovych, became president last year.
Notable deaths of 2014 [PHOTOS]
Bus attack in Pakistan kills eight