Any foreigner working in Thailand must obtain a Work Permit before beginning work. While a prospective employer may file an application on the foreigner's behalf in advance of his commencing work, the actual Work Permit will not be issued until the foreigner has entered Thailand in accordance with the immigration laws and has presented himself to receive his Work Permit. 

The Permit initially will be valid only for the period of the foreigner's Non-Immigrant Visa permits him to remain in Thailand under the Immigration Law. The Work Permit will be subject to renewal in accordance with the renewed or extended visa. For foreigners who are holders of a Thai Certificate of Residence, the Work Permit can be renewed annually. The Labor Department, subject to subsequent renewal, will in principle grant an initial duration of one year for the Work Permit. A Work Permit must be renewed before its expiry date or it will automatically lapse.

• You must have a work permit to perform any type of work in Thailand. This also includes volunteer and non-profit work.
• To obtain a work permit you must have a company employ and sponsor you. An individual cannot simply apply for a work permit on their own.
• A work permit is only valid for the company that employs you. You CAN work for multiple companies in Thailand, but you must hold a valid work permit for each.
• If you do not have a company to employ/sponsor you for the work permit, you can form your own Thai company and obtain the required permit. .
• You need to employ 4 Thais per 1 foreigner to apply a work permit.
• You Must have a Non-Immigrant visa to apply for a work permit.

Warning: The penalty for working in Thailand without a work permit can be from 3 months to 5 years imprisonment. You will also face deportation and be blacklisted from Thailand for 99 years.

Work Permit Requirements:
Required Documentation from the individual
The following documents must be attached to a Work Permit application

•  For non-permanent residents: A valid passport containing a Non-Immigrant visa.
•  CV or Resume showing application's educational qualifications and describing in detail the applicant's past position, duties, performance, and place and length of employment.
•  A recent medical certificate from a first-class licensed physician in Thailand stating that the applicant is not of unsound mind and not suffering from leprosy, acute tuberculosis, elephantiasis, narcotic addition or habitual alcoholism.
•  Three 5x6 cm. full-faced, bareheaded, black and white or color photographs, taken no more than six months prior to the filing of the application.
•  If the job applied for is subject to a license under a particular law, in addition to the Alien Occupation Law, a photocopy of such license, (e.g. teacher's license, physician's license, press card from the Public Relations Department, certificate of missionary status from the Office of Religious Affairs, etc.) shall be attached.
•  If the applicant is married to a Thai national, the original and photocopies of the following must be presented: Marriage certificate, spouse's identity card, birth certificates of children, household registration, as well as photocopy of every page of the applicant's passport.