The drugs were divided in 487 packages and hidden among asphalt rollers the ship was carrying, prosecutor Vladimir Chavdarov.
Customs officials in the Black Sea port of Varna valued the seized heroin at 18 million euros ($22 million).
Two Bulgarian men - the owner of the receiving company and a customs officer - were detained and could face up to 20 years in jail, if convicted on drug trafficking charges.
Bulgarian police believe the heroin was not intended for the domestic market but rather destined to be sold in Western Europe.
Drugs from the Mideast sometimes are transported on trucks from Afghanistan through Iran, Turkey and Bulgaria to Western Europe.
Bulgaria has taken steps in recent years to prevent drug trafficking.