This is a direct copy of the USA Embassy advice. It can also be relied upon by most other nationalities. The USA example has been chosen because it is so well laid out.
Visa Exempt Entry
U.S. citizens carrying a tourist passport and in possession of an onward or return airline ticket do not require a visa to enter Thailand. The passport must have at least six months validity remaining to be allowed entry. Upon entry, Thai immigration officials will place an immigration stamp in the passport permitting a 30-day stay in Thailand if arriving by air or a 15-day stay if arriving by land. This time limit may subsequently be extended for an additional 7 days by paying a 1,900 baht fee to the Thai Immigration Bureau office. The headquarters of the Thai Immigration Bureau is located at Government Center Chaengwattana Building B, No. 120, Moo 3, Chaengwattana Road, Tungsonghong Sub-District, Laksi District, Bangkok 10210, 66-2-287-3101 through 66-2-287-3110. For more information or additional Thai Immigration Bureau locations, please consult the Thai Immigration Bureau website.
NOTE: The Embassy recommends that U.S. citizens carrying a diplomatic or official passport obtain a visa prior to arriving in Thailand. It is sometimes possible for diplomatic or official passport holders to enter without a visa, but it is up to the discretion of the airline and/or immigration personnel and you may be denied entry.
If an individual wishes to remain in Thailand for more than 30 days, he/she may wish to obtain a tourist visa at the Royal Thai Embassy or Consulate in the United States, prior to arriving in Thailand. The tourist visa must be used within 90 days from the date of issue and allows an initial stay of 60 days. After arrival in Thailand, a tourist visa may be extended twice, each time for an additional 30 days. There is a 1,900 Baht fee for each extension. Interested Americans should contact the Thai Immigration Bureau located at Government Center Chaengwattana Building B, No. 120, Moo 3, Chaengwattana Road, Tungsonghong Sub-District, Laksi District, Bangkok 10210, 66-2-287-3101 through 66-2-287-3110. For more information or additional Thai Immigration Bureau locations, please consult the Thai Immigration Bureau website.
American citizens who wish to remain in Thailand for longer than 90 days during any six-month period will be required to obtain a valid Thai visa from a Thai Embassy or Consulate that is authorized to issue visas. The U.S. Embassy advises all American citizens who wish to obtain a Thai visa to contact the Thai Immigration Bureau for exact visa requirements and regulations. Persons who do not comply with visa regulations risk being denied entry to Thailand at the border.
All foreigners interested in working in Thailand must obtain a Thai work permit and a Thai work visa. In order to receive a work permit, a company, foreign government, or other organization in Thailand must file an application on the behalf of the work visa applicant. Once obtained, the work visa is valid for one year. For more information, please consult the Ministry of Foreign Affairs or the Royal Thai Embassy in Washington, D.C. websites.
Retirement visas for Thailand are available to foreigners 50 years of age or older. These visas are valid for only one year and employment of any kind is strictly prohibited. According to Thai immigration, applicants must meet the following qualifications:
•Applicant must be able to provide proof of a pension or other regular income from a source outside of Thailand;
•Applicant’s pension or other regular income must be no less than the equivalent of 65,000 Baht per month;
•Alternatively, the applicant may meet the financial requirement by maintaining a Thai bank account with a minimum amount of 800,000 Baht. (Applicants will need to show that they have 800,000 Baht in savings each year when they renew their visa.)
•Any applicant married to a Thai citizen may be able to receive a visa on that basis rather than retirement.
For additional requirements and information, please consult the Ministry of Foreign Affairs or the Royal Thai Embassy in Washington, D.C. websites.
If an individual does not depart Thailand prior to expiration of his/her airport permit or visa, the individual is in the country illegally. In order to depart Thailand, it will be necessary to pay an overstay fine. The fine for overstaying a visa is 500 Baht per day, up to a maximum of 20,000 Baht. The fine is payable to the Immigration Bureau, the Suvarnabhumi Airport Immigration office, or other departure point.
Thai police have been known to sweep areas frequented by low-budget travelers and arrest those who have overstayed their visas. Those arrested may be held in the Immigration Detention Center until they can pay for their fines and pay for tickets out of Thailand. Those who overstay their visas for more than 200 days may be required to serve time in the Immigration Detention Center before they are deported. It is highly advisable to avoid visa overstays.
Expired short-term visas can be extended after paying a fine, but the number of days elapsed since expiration is subtracted from the extension period. Application for extension must be made by the next working day after visa expiration to avoid a fine.
Please note: applications for extension cannot be made on Saturdays, Sundays, and Thai holidays.
Thailand’s entry/exit information is subject to change without notice. For further information on Thailand’s entry/exit requirements, contact the Royal Thai Embassy, 1024 Wisconsin Avenue, N.W., Washington, D.C., 20007, telephone 202-944-3600, or contact the Thai consulates in Chicago, Los Angeles, or New York City. Visit the Royal Thai Embassy website for the most current visa information.