Police said 26 workers were crammed into the vehicle after their shift when the accident happened late yesterday in the coastal province of Samut Sakhon, 45 kilometres from Bangkok.
"It should not have happened... The pick-up truck had just left the factory and was not travelling far to its destination," Chaiyuth Thomya of Samut Sakhon's Mueang district police told AFP, putting the death toll at nine.
Thailand's roads are among the world's deadliest, with accidents involving buses and trucks packed with migrant workers particularly common.
The kingdom, the world's third-largest seafood producer, is under intense pressure to raise standards in the industry, with the European Union threatening a ban on its exports if it fails to clamp down on illegal fishing.
Thai authorities have vowed to get tough with offenders, saying any export ban to Europe could cost the kingdom around a USD 1 billion a year.
Rights groups also accuse Thai authorities of failing to protect migrant labourers in the notoriously unregulated fishing industry, which is blighted by accusations of slave labour, low pay and poor conditions.