It happened when the unit was returning to the northern Malaysian city of Ipoh after guarding a Hindu festival, Perak Emergency Services said in a statement.
"A police truck collided with a truck carrying gravel, trapping three victims in the front with 15 others in the back," the statement said.
Initial reports said eight members of Malaysia's Federal Reserve Unit were killed, plus the driver of their vehicle. A ninth paramilitary member later died of his injuries, news reports said.
"Two others remain in critical condition and a further seven are being treated for moderate injuries," Perak police chief Bakri Zainal Abidin told the New Straits Times newspaper.
Police arrested the 40-year-old lorry driver in connection with the accident, he added.
Malaysia has one of the highest road accident death tolls in Southeast Asia, with somebody dying in a car accident almost every two hours, The Star newspaper reported in March.