This is frowned upon and in fact illegal (though of course the law is seldom enforced - unless drunk and bad-mouthed too). the max fine is actually 500baht! For women, when walking around - do wear a bra. Thais frowns upon western women they see bra-less.

While there is no legal requirement to wear a bra in Thailand, Thai culture places a very high value on modesty and "keeping face" in public. In major cities like Bangkok and when visiting any religious site, appearing "bralless" in thin or revealing clothing may be viewed as disrespectful or overly provocative. Thais generally dress quite conservatively; even in the sweltering heat, many locals prefer light, breathable fabrics that cover the shoulders and knees. If you are at a beach resort or a heavily touristed island like Phuket, the dress code is much more relaxed and "Westernized." However, if you plan to visit a Buddhist temple, a bra (or a supportive camisole) is considered essential as part of the requirement to be fully and modestly covered. To balance comfort with cultural respect, many travelers opt for lightweight sports bras or linen tops that aren't transparent, ensuring they don't draw negative attention while navigating local markets or public transport.