The former prime minister was freed from prison on Saturday, after the Parliament abandoned President Viktor Yanukovych and ordered her release.
Speaking to crowds outside the building, the fiery 53-year-old announced her intention to run for president, saying she will "make it so that no drop of blood that was spilled will be forgotten."
Ms Tymoshenko after being released from jail
Her announcement means Ms Tymoshenko has squarely positioned herself as a figurehead for the second Ukrainian uprising in 10 years.
The pro-Western opposition leader was a key figure during Ukraine's 2004-2005 Orange Revolution, which overturned a rigged election won by Mr Yanukovych.
Famed for her trademark blonde hair worn in braids, she served as prime minister from 2007-2010, only to be forced out of office after Mr Yanukovych beat her to the presidency.
Ms Tymoshenko was sentenced in 2011 to seven years in prison for abuse of office over a gas deal with Russia.
Yanukovych and Tymoshenko have long been arch rivals
Her opponents describe her as a political opportunist who benefited enormously from widespread corruption in the 1990s.
But her supporters and Western backers claim her imprisonment was politically motivated.
They back her pro-European stance in a country which is split between those who favour an allegiance with Russia and those who want to improve ties with Europe.
Yevgenia Tymoshenko reacts as the Parliament votes to free her mother
Her polarising reputation has earned her the nickname "Iron Lady", or simply the Ukrainian word for "she" - "vona".
She spent much of her confinement in hospital in the northeastern city of Kharkiv, where she was treated for a severe back injury.
Ms Tymoshenko also frequently alleged she was abused in prison and had sought medical care in Germany.
