The blast at a grain depot at the port of Derince on the northern shore of the Gulf of Izmit sent up plumes of thick smoke and dust.
An initial investigation showed the explosion was caused by “wheat dust compression during the transfer of wheat from a ship to the silo," said Seddar Yavuz, governor of Kocaeli province.
The explosion at the Turkish Grain Board (TMO) site was still being investigated, he added.
Kocaeli Mayor Tahir Buyukakin said three of the injured were hospitalized in serious condition. All personnel had been accounted for.
The Transport Ministry said no ships were affected.