Turkey's Disaster and Emergency Management Authority said the earthquake struck in the Sea of Marmara at 1:59 p.m. (1059 GMT) at 7 kilometers (4.4 miles) deep and was felt throughout the western Marmara region, which includes Istanbul.
Istanbul mayor Ekrem Imamoglu said on Twitter there were no reports of deaths from the quake, ABC news reported.
News videos showed a collapsed minaret in the city's western Avcilar district and the emergency agency said cracks were found in some buildings. Turkish media showed children being evacuated from schools and city residents waiting outside their homes. Schools were cancelled for the day.
The Kandilli Observatory and Earthquake Research Institute recorded several aftershocks, with the highest at 4.4 magnitude.
Turkey is crossed by fault lines and prone to earthquakes. Experts have long warned that a major earthquake is expected to hit Istanbul, Turkey's most populous city with more than 15 million residents. A 4.6 magnitude earthquake hit the city on Tuesday.
In 1999, a 7.4 magnitude earthquake in western Turkey killed more than 17,000 people.