By law, land for agriculture is restricted to Thais only.
Deputy Commerce Minister Alongkorn Ponlaboot said the Department of Special Investigation (DSI) and the Business Development Department had found foreigners owning tracts of land in the North and Central provinces.
"The government has found that some foreign companies have purchased, through brokers, vast territory for rice farming. Some small areas of land are also in foreigners' hands, mostly through marriages to Thai women," said Alongkorn.
The ministry has not named the companies or nationalities for fear of compromising the investigations.
To protect Thai rice-farming businesses and land ownership, Alongkorn said the government would continue to examine whether the Foreign Business Act had been violated.
The DSI will consider taking up the case if it finds foreign firms had intentionally violated the law, Alongkorn said.
The Business Development Department has been investigating documents and monitoring foreign companies' movements for action that might conceal their business objectives.
Alongkorn said he had also ordered related agencies to investigate land owned by Thai women, to see if they are acting as nominees for foreigners engaged in farming businesses.
There have been rumours that foreign investors, especially from Taiwan and the Middle East, have invested in rice farms in the Central and Northern provinces, raising concerns among Thai farmers and authorities.
Despite this stringent enforcement of the Foreign Business Act, Alongkorn said the Kingdom still welcomed foreign investors.
As a major world food crop producer, the Thai government has been prompt in signing trade contracts with any country wanting to ensure its food security, he said.