The vehicle, packed with explosives, went off as a bus carrying at least 12 Chinese engineers and some Pakistanis to a port project at Gawadar - about 500km west of Karachi -passed by.
"A car was parked near the port and it exploded as the bus reached the port," said Sattar Lasi, the chief of police in Gawadar. The attack occurred just after 9:00am (04:00 GMT).
Nine of those injured were Chinese. The others were a Pakistani driver and security guard, said Lasi. Three were in a critical condition.
He added that authorities had not found the remains of any bomber inside the white Suzuki car, though the force of the blast would make it hard to determine whether someone was inside.
The motive for the attack was not immediately clear.
The Chinese all worked for the China Harbour Engineering Company in a project to develop the Gawadar seaport on the Arabian Sea. It was not clear how long they had been in the country.
The project at Gawadar is to develop a deep-sea port that will ultimately provide a second option to the heavily trafficked Karachi port terminal.
China is Pakistan's most important ally and one of its chief trading partners.