India-based Tata said in a statement it "deeply regrets to announce the untimely and tragic" death of Mr Slym.
The circumstances of his death were not immediately apparent, although several media reports said he may have had a fall at a hotel in the Thai capital.
Aged 51, the British-born executive ran all Tata Motors' operations except Jaguar Land Rover in the UK.
Mr Slym had worked for Toyota in the UK, and then General Motors in India and China. He had been managing director of Tata Motors, part of the giant Tata Group, since October 2012.
Although Jaguar Land Rover has been hugely successful, the rest of Tata Motors has struggled.
Mr Slym was brought in to help overhaul the manufacturing, sales and distribution operation, including Tata's new-look Nano ultra-cheap car and building a new generation of engines.
In his tribute, Tata chairman Cyrus P Mistry described Mr Slym as  "a valued colleague who was providing strong leadership at a challenging time for the Indian auto industry".
"In this hour of grief, our thoughts are with Karl's wife and family," Mr Mistry said.
Tata Motors' managing director Karl Slym has died in Thailand where he was attending a board meeting.