Giuliana Sgrena, who writes for the leftist daily Il Manifesto, was abducted just after calling her office to say she was fine, on Friday.
"Giuliana called us to say she was fine, but five minutes later her interpreter rang us back to say that she had been abducted near a mosque in Baghdad," said Gabriele Polo, one of the daily's directors.
He said it appeared that an armed gang fired on the car she was travelling in, forcing it to stop. She was seized and driven off.
Polo, whose paper opposed the US-led invasion of Iraq in 2003, said US and Iraqi forces and the Italian embassy had been informed of the kidnapping.
An Italian journalist, Enzo Baldoni, was kidnapped on 20 August between Baghdad and Najaf. He was killed by the Islamic Army in Iraq and his death shown on a video released over the internet.
In September, two Italian women working for humanitarian organisations were abducted and held for three weeks before being released.
A mosque in central Baghdad
near where Sgrena was last seen
About 20 foreigners seized since Iraq's hostage crisis began in April remain missing. Groups claim to have murdered more than 30 foreign captives, but the authenticity of some claims has been questioned.
Florence Aubenas, a 43-year-old senior correspondent for the left-leaning French newspaper Liberation, and her Iraqi translator Husain Hanun al-Saadi have not been heard from since they left her Baghdad hotel on 5 January.
Elsewhere in Iraq, a roadside bomb killed three civilians driving in a truck carrying vegetables at Ishaki, about 100km north of the capital, police said.
Two other civilians were killed on Friday by a booby-trap bomb as they drove behind an Iraqi army convoy.