The attack on Tatyana Chornovol came after she posted photos online of what she said was a top government official's luxury estate, RIA Novosti reported.
Chornovol was driving to her home in Kiev about 1:30 a.m. local time when a car cut her off and forced her off the road, a police statement said. Men jumped out of the car, dragged Chornovol out of her vehicle and beat her before leaving her in a ditch.
She was hospitalized with a concussion. Images posted online showed the journalist with the right side of her face and lips swollen and bloody.
Ukrainian police said they are investigating the attack.
Opposition leader Anatoly Gritsenko said a dashboard camera in Chornovol's car captured the make and model of the attackers' car, the report said.
Chornovol also reported on Ukraine's protest movement, which has grown since November after President Viktor Yanukovych backed out of an association agreement with the European Union.
Another opposition activist, Dmitry Pilipets, was stabbed four times late Tuesday night by two attackers. He has been hospitalized in stable condition.