They said the 18 are employed by Nurol Insaat, a Turkish construction company contracted to build a sports complex in the sprawling Shiite district of Sadr City. The kidnappers stormed the construction site, where the workers were sleeping in caravans, breaking down doors and disarming the guards before taking the workers away.
The Iraqi officials said an Iraqi national was kidnapped along with the Turks.
Turkish Foreign Ministry Spokesman Tanju Bilgic said those kidnapped included 14 workers, three engineers and one accountant.
There were no reports of violence, the Associated Press reported.
Turkish Deputy Prime Minister Numan Kurtulmus confirmed the kidnapping and said Ankara was in close contact with authorities in Iraq. "The Iraqi authorities for the time being do not have information on how the incident occurred or who captured them," he told reporters.
In Baghdad, Interior Ministry spokesman Saad Maan told The Associated Press that authorities are investigating the incident and that a contingent of security forces has been tasked with tracking down the kidnappers.
Neither the identity nor the motives of the kidnappers were immediately known.
Associated Press footage of the site taken hours after the kidnapping shows the sports complex to be almost complete. A sign outside says it includes a 30,000-seat soccer stadium, a track and field facility and a 50-room hotel.
Turkey recently began launching airstrikes against ISIL in Syria and allowing US warplanes to use bases in southeastern Turkey to strike the extremist group.
It launched a simultaneous air campaign in northern Iraq against the Kurdistan Workers' Party, or PKK, a Kurdish militant group.
Baghdad has been torn by violence for over a decade now, with roadside bombs, suicide attacks and assassinations occurring almost daily. While kidnapping for ransom has continued, abductions on the scale seen Wednesday have been almost unheard of in the past few years.