Mae Sai is the northernmost town at the northermost geographical point in Thailand at the border to Burma/Myanmar. It's part of Chiang Rai Province and part of the Golden Triangle. There is a lively border crossing between the two countries here.
The place lies at the end of Phahonyothin Road, one of Thailand's biggest highways, what is also Asian Highway no. 2 (AH2). It's about 260km north of Chiang Mai and about 850km away from Bangkok. Mae Sai concentrates along this road, although the fast growing place spread out in the recent decades along many sideroads. It's a trade point for goods of different kinds from Burma/Myanmar. The shops are crammed with cheap, counterfeited electronics, cigarettes and peculiarities one does not see elsewhere in Thailand. On the sidewalks sit many salespeople who present their goods in outdoor shops. Women run small stalls on two wheels on which they toast chestnuts from Burma/Myanmar, who they sell. Touts frequently ask tourists to buy their faked viagra. Mae Sai is, although a small place, a busy place.
Of course, these border towns are always a bit strange, and a peculiar atmosphere lies in the air. Smuggling happens. At the banks of Ruak River, the border river dividing Thailand and Burma/Myanmar, are a few restaurants with a view over the river, close to the only bridge which connects both sides.
Sitting there and having lunch, I saw locals crossing the river in both sides. Those who came from Burma/Myanmar carried boxes on their heads, targeting Thailand. Above them the bridge with a number of border officials in uniforms. While monitoring all what's going on on the bridge, the officials never looked down on what happened in the river below them.
The place on the border's other side is Tachileik in Shan Province. It's possible for Westerners to get a visa for a fortnight there for ten dollars respectively 500 baht. In the past it was always a point for the notorious visa runs, but after what I read recently, that is no more possible since 2016?! The Thai immigration administration is frequently changing the rules. And you never know what's coming next...
Local people frequently cross the border mostly for business reasons. Thai People come to make a short trip into Burma/Myanmar.
Mae Sai is itself of little interest. There is no interesting building in the place, no historical significance of it. Coming here for buying unusual things from Burma/Myanmar, one has to keep in mind that most of it is really cheap crap in the sense of low quality.
While the scenery around town is mostly flat plains in the east along the Ruak River, west of town stretches the Daeng Lao Range (see also the article on Fang for that). These mountains are quite steep and high here and give quite an impression when approaching on the road from Chiang Saen. When passing through the market which has a big, turquoise roof and following the road upwards, one can climb up to Doi Tung (Mountain). On top of the mountain is, as so many, many times in Buddhist countries, a temple placed.
There is a frequent bus connection between Mae Sai and Chiang Rai, what is about 60km south. Idiotically, the bus station lies about 10km outside town at the easter side of highway no. 1. For the (un)necessary extra transport, Songthaews commute between the border and the bus station.