What's the penalty for driving without a Thai licence?

Under Section 64 of the Motor Vehicle Act B.E. 2522 (1979), driving a motor vehicle without an appropriate licence is punishable by a fine of up to THB 1,000 for a first offence, or up to THB 2,000 and/or one month imprisonment for driving the wrong class of vehicle (e.g. car licence used to operate a motorcycle).

Critically, the fine is the least of your worries:

• Your insurance is void. Under standard Thai motor policies and Section 7 of the Compulsory Motor Vehicle Accident Victims Protection Act B.E. 2535 (1992), driving without a valid licence voids both your third-party comprehensive policy and your own bodily injury cover. The compulsory CMI (พ.ร.บ.) policy that all vehicles carry still pays out to the victim — but the insurer will then seek recovery directly from you.
• Your visa may be affected. A criminal record, even for a minor traffic offence, must be declared on visa extension forms and can be grounds for refusal.
• Hospital bills. A serious accident with no insurance can run to THB 1–5 million.

International Driving Permits (IDP). Thailand accepts IDPs issued under the 1949 Geneva Convention. The 1968 Vienna Convention IDP is not universally accepted by Thai police, despite being more common in Europe. Check your IDP before you travel and renew it through your home country’s automobile association — Thailand cannot issue one for you.

Converting to a Thai licence. If you have a non-immigrant visa and a Certificate of Residence from your embassy or immigration office, you can convert your home licence at any Department of Land Transport office (กรมการขนส่งทางบก) for a fee of around THB 200–300. The process takes about half a day and involves a colour-blindness test, reflex test, and a video lesson — no road test if you hold a valid foreign licence.