Judge Rodion Kireev said Tymoshenko, a leader of Ukraine's pro-Western Orange Revolution, abused her office by personally issuing directives related a gas deal with Russia in 2009, RIA Novosti reported.
Kireev also ordered Tymoshenko to pay $189 million in compensation for losses incurred by Naftogaz, Ukraine's state energy supplier.
Tymoshenko said she would challenge the ruling, saying the judge was "taking Ukraine's freedom" even as Kireev read his decision.
Tymoshenko said she would ask the European Court of Human Rights to "defend her honor" and decide whether the conviction resulted from a grudge by President Viktor Yanukovych. She maintained the trial was politically motivated and accused Yanukovych, her rival during the Orange Revolution, of being behind her prosecution.
"It is not Judge Kireev but President Yanukovych who is handing down the verdict," she said.
Tymoshenko supporters from her party were barred from rushing the courtroom, RIA Novosti reported. About 800 Tymoshenko supporters and 1,000 anti-Tymoshenko demonstrators were outside the court in Kiev.
A Ukrainian opposition leader said a guilty verdict wouldn't bode well for Ukraine's hopes for associate European Union membership.
"If there is a guilty verdict Ukraine will not be able to sign an association agreement, and we will turn into something like Belarus," Arseniy Yatsenyuk, leader of the Front of Change party, told supporters.