Tetyana Chornovol said she was stepping down as head of Kiev's National Anti-Corruption Committee, just six months after being appointed amid widespread hopes of a clampdown on Ukraine's endemic corruption after the ouster of former leader Viktor Yanukovych.
"Ukraine does not have the political will for an uncompromising and large-scale war against corruption," she wrote in a blog announcing her decision.
Chornovol, who was severely beaten in December after she took a leading role in the anti-Yanukovych protests, said that her time in charge of the anti-graft body had been "in vain".
A draft law to purge officials with links to Yanukovych's infamously corrupt administration has been stalled by lawmakers fearful that they may be targeted for prosection.
Chornovol's husband, Mykola Berezovy, was killed August 10 fighting for a volunteer battalion against pro-Russian rebels in the east of the country.