About 1.5 tonnes of elephant ivory; 80-100 slaughtered elephants have been seized, the authorities in the Democratic Republic of Congo said.
It is one of the largest hauls in Africa in recent years.
Three years ago, Vietnamese officials discovered over nine tonnes of elephant ivory hidden in a timber shipment from Congo-Brazzaville.
In 2013, Kenyan officials made several seizures including one of four tonnes. Togolese authorities likewise seized four tonnes of ivory over the course of one week in 2014.
The haul was discovered on trucks in the south-eastern city of Lubumbashi, but the origin and intended final destination of the ivory remain unclear.
International ivory trading has been banned since 1990 but demand remains high in China and south-east Asia, where African ivory is used in traditional medicine.
Police said they arrested five people, though two of them fled during questioning.
