The deceased included two minor children, aged 13 and 10, police said.
While four passengers died on the spot, another passed away in Kozhikode Medical College Hospital, where all the injured were admitted, they said.
The mishap occurred when the bus tried to overtake the lorry, swerved to avoid an oncoming car and rammed into the rear of the truck, police said.
The passengers, all from Mattanur in Kannur district, were returning from Salem in Tamil Nadu after attending a wedding, they said.