The Interior Ministry said initial information suggests the lorry, which was found on a highway in Sofia, was carrying around 40 migrants, and the survivors were taken to nearby hospitals for emergency treatment.
Three people have been detained following the discovery of the truck and have not yet been charged, Investigative Service Chief Borislav Sarafov said.
The migrants’ nationality is currently unknown, though some local media has identified them as Afghanis.
Bulgarian health minister Assen Medzhidiev said the survivors were in very bad condition.
He said: “They have suffered from lack of oxygen, their clothes are wet, they are freezing, and obviously haven’t eaten for days.”
The driver was not with his vehicle, which was found near the village of Lokorsko, 12 miles (20km) to the northeast of the capital.
Police discovered the passengers hidden in a secret compartment underneath a cargo of timber. Eight of the migrants are reported to be in very serious condition.
Bulgaria is a gateway for migrants coming into the European Union through Turkey. The government has been accused of mistreating refugees as they attempt to enter the continent.
Sky News reported last year that officials near the Bulgaria-Turkey border fence had shot an asylum seeker in October after they caught a group of people illegally entering the country.
Bulgaria is part of the European Union and hopes to get membership in the Schengen area, which allows people to move freely across borders within it.
Last year, more than 150,000 people attempted to cross the border with Turkey, which was more than four times higher than the previous year.