Her release sends shockwaves through Ukraine's politics, at a moment of deep turmoil following the worst unrest in the sprawling country since the days of the Soviet Union.
The 53-year-old, instantly recognisable for her peasant-style braided blonde hair, shot to fame with her impassioned speeches against a rigged poll won by Viktor Yanukovich in 2004.
With the post-Soviet establishment turfed out, Tymoshenko became prime minister, but fell out with her fellow Orange revolutionaries.
Ousted after Yanukovich beat her to the presidency in 2010, she was jailed in 2011 over a gas deal she signed with Russia. She spent much of her incarceration in hospital for a back problem in the northeastern city of Kharkiv.
The West said her imprisonment was politically motivated, and the European Union repeatedly pressed for her release, only to be rebuffed by Yanukovich.
Tymoshenko is reported to have made millions in the 1990s as president of a company that was for a while the main importer of Russia natural gas, earning her the sobriquet "Gas Princess".
Her sharp tongue and combative style brought her both devout followers and arch enemies.
Born in 1960, in Russian-speaking Dnipropetrovsk in eastern Ukraine, Tymoshenko studied at the local university, married while still a teenage student and had a daughter in 1980.
Taking advantage of an entrepreneurial climate in the Soviet Union under leader Mikhail Gorbachev, Tymoshenko had her first taste of self-made money from a video rental store she set up.
She soon crossed into the energy sector and went on to become head of Unified Energy Systems.
Tymoshenko entered parliament in 1996 and was made a deputy prime minister in 2000. She fell victim, however, to political intriguing under President Leonid Kuchma and spent several weeks in jail on corruption charges. She was cleared.
On leaving prison, she changed her image, lightening and braiding her hair and wearing sharply tailored, folk-inspired designer outfits. Her stylist told media the look was designed to distance herself from an association with wealth and to emphasise a national Ukrainian identity.
She is now immediately catapulted to the forefront of Ukrainian politics, but will find the stage more crowded than before, with a new generation of revolutionaries who led the mass protests that appear to have brought down Yanukovich. (Writing by Matt Robinson; Editing by Peter Graff)