An Iraqi police colonel said the Turks were kidnapped by black-clad gunmen in pickup trucks, and that authorities had formed a committee to investigate their abduction.
Turkish foreign ministry spokesman Tanju Bilgic said those seized were three engineers, an accountant and 14 workers.
"We have learnt that the Turkish workers had been separated from workers from other countries during the kidnapping and were specifically chosen," he was quoted as saying by the state-run Anatolia news agency.
Nurol Insaat, a leading Turkish construction company, confirmed in a statement to AFP that its workers had been abducted in Baghdad and said that it had not yet received any ransom demand.
Nurol's website said it had been awarded a "design-build contract" for a 30,000-seat Sadr City Stadium, and that the project includes practice fields and a hotel.
Kidnappings for ransom are a persistent problem in Baghdad, and the identity of the kidnappers was not immediately clear.
A political motive is also possible, as Sadr City is a stronghold of pro-government Shiite paramilitary groups battling IS, which overran large areas of Iraq last year.
- Kidnappings targeting Turks -
Ankara has been accused of complacency towards IS and complicity in assisting the jihadist group, which also holds substantial territory in Syria, just over Turkey's southern border.
Turkey has carried out limited strikes against IS in Syria, but in an indication of its priorities, has directed far more firepower against the rebel Kurdistan Workers' Party (PKK).
The latest kidnappings were the third time a large group of Turks has been seized in Iraq over the past year and a half.
IS abducted 46 Turks from the country's consulate when the militants seized the northern city of Mosul in June 2014, as well as more than 30 Turkish truck drivers.
Both groups of captives were later freed.
Turkey is a major trade partner for Iraq, especially with the country's autonomous Kurdish region.
But the relationship between Ankara and Iraqi Kurdistan has been a source of tension with Baghdad, which considers the autonomous region's independent export of oil via Turkey to be illegal.
Relations between Baghdad and Ankara have been rocky for years, with the two sides also at odds over the Syrian civil war.
But ties have improved somewhat since Prime Minister Haider al-Abadi took office last year, replacing Nuri al-Maliki, who had repeatedly clashed with Ankara.
Baghdad turned to mostly Shiite volunteer forces for support as IS advanced towards the capital in June last year, and they have played a key role in halting and then reversing the jihadists' gains.
But in doing so, the Iraqi government empowered Shiite militias, some with chequered human rights records, and spurred the creation of new ones, allowing them to act with near-impunity.
Some fighters linked to the militias stand accused of engaging in criminal activities, including kidnappings as well as property seizures and murders.