The warnings have been issued since anti-government protesters launched a campaign to force the government's ouster in October last year, the Foreign Ministry says.
Eight countries have suggested their citizens leave Thailand or avoid travelling to the kingdom unless it is necessary.
Kuwait and Iran have advised their nationals to leave Thailand. Kazakhstan, Oman and Russia have warned their citizens to avoid coming to Bangkok, the main protest area.
Malaysia, United Arab Emirates and Qatar have suggested their people postpone visits to Thailand.
The Constitutional Court last week disqualified Yingluck Shinawatra as caretaker prime minister.
Many countries said they viewed the ruling as likely to increase tensions, the ministry said.
Five countries – the United States, the United Kingdom, Canada, Italy and France – have also warned people to be cautious about travelling here after an earthquake and frequent aftershocks hit Chiang Rai province from last week.