"We are starting to see more and more homeless foreigners, many of whom have separated from their Thai wives and now have no money," said Natee Saravari, secretary-general of the Issarachon Foundation.
While foreigners can own condos, houses and other property are usually registered in their wives' or girl friends' names, meaning they have no rights and can be ejected.
The Thai charity has been helping homeless Thai people in Chiang Mai, Chon Buri and Phuket provinces for the past 10 years, but has recently branched out into assisting homeless foreigners as well.
"In Pattaya we see them sorting through the trash in front of McDonald's for something to eat, and hanging out in front of restaurants asking customers for money," Mr Natee said.
He estimated that there were more than 200 homeless foreigners nationwide, compared with about 30,000 homeless Thais.
"As many as 40% of the Thai homeless suffer from mental illness, but most of the foreign homeless are alcoholics," he said.
The foundation has urged the Ministry of Foreign Affairs to address the growing problem by setting up a system with embassies to cope with their penniless nationals, most of whom are living in Thailand without passports or on expired passports.
Thailand attracted more than 22 million tourists last year and has a growing population of Western retirees living in the kingdom on a permanent basis.
"Thailand's laws covering foreigners are very outdated and should be revised to deal with the changed circumstances," said Buaphan Promphakping, an associate professor of social studies at Khon Kaen University.
"Under the current laws, foreigners' rights are not very well protected," said Buapan, who has done research on the growing number of foreigners married to Thai women in northeast Thailand and on the kingdom's expanding population of Western retirees.