Coast guard officers seized just over 100 kilos of heroin at the northwestern port of Igoumenitsa on Saturday.
Authorities said the drug was hidden in special compartments in the truck, which was being used to transport livestock.
The vehicle was searched before it was due to board a ferry for the Italian port of Ancona.
Two men, Bulgarians aged 35 and 37, were arrested.
Officers said that heroin, weighing 100.2 kilos, was divided into 190 packages and was discovered by a sniffer dog.