Is working remotely on a tourist visa in Thailand actually illegal?

Yes. A Thai tourist visa is issued for tourism and does not authorise work. While some people believe working remotely for an overseas employer is acceptable, Thai law does not provide a general exemption for remote work on a tourist visa, which means it can create immigration and employment law risks.

What We See In Practice

I'm only working on my laptop for my overseas employer. Surely that doesn't count as working in Thailand?

Not necessarily. Many people assume that if they’re paid overseas and have no Thai clients, they’re free to work from Thailand on a tourist visa. Unfortunately, the law isn’t that simple. Thailand has a broad definition of “work”, and there is no general exemption for remote work carried out on a tourist visa. To be able to work, the correct visa (usually the Non-Immigrant B visa) and a work permit is required.
A tourist visa is intended for people visiting Thailand for tourism, not for work. If you plan to work while you are in Thailand, you should normally hold the appropriate visa and, where required, a work permit or other form of work authorisation. If you want to work remotely from Thailand, it is usually better to use a visa designed for that purpose, such as the Destination Thailand Visa (DTV) or, if you qualify, the Long-Term Resident (LTR) Visa, both of which are intended to accommodate remote work arrangements.
The biggest risk is assuming that being paid overseas makes the work legal. In practice, Immigration and the Department of Employment will look at the overall circumstances, including the type of work being carried out and your activities while you are in Thailand. If you plan to work remotely from Thailand for more than a short holiday, it is usually better to use a visa designed for that purpose, such as the Destination Thailand Visa (DTV) or, if you qualify, the Long-Term Resident (LTR) Visa.