The explosion, at around 11:30pm local time (3:30pm GMT), was reported to have come from a shipment of explosives in a container terminal at the Binhai New Area industrial zone.
The cause of the explosion is currently unknown.
Citing the nearby Taida hospital, Xinhua News said 50 people had been injured and “new ones keep coming”. The injuries were mainly from broken glass and stones, the report said, with some “serious” injuries.
Residents in neighbourhoods near the explosion reported that the flame lit up the sky and the blast blew dust “dozens of metres high”. Window glass was also reported to be shattered.
A number of nearby tower blocks are without power, Chinese state broadcaster CCTV said. Xinhua said shockwaves from the blast were felt “kilometres away”.
Tianjin is a key industrial and port city in China, located 70 miles to the south of Beijing.