Can I eat it only in Chiang Mai?
No. It is a signature dish eaten in many northern cities. If you are meeting northern food for the first time in Chiang Mai, you could start with a bowl of khao soi.
Khao Soi · ข้าวซอย
If you are choosing a meal in Chiang Mai, try khao soi. It is a signature northern dish of egg noodles in a coconut curry broth, topped with crispy fried noodles. There is fun in alternating soft noodles and crunchy noodles. If lime comes on the side, taste the broth first and then squeeze in a little.

The rich curry broth and the texture of two kinds of noodles are its character. The noodles on top are crunchy at first and slowly soften as they touch the broth. Taste the broth before and after adding the accompaniments, and you can enjoy different flavours in one bowl.
Try the crunchy noodles on top first, before they get wet in the broth.
Khao soi is a well-known dish on a Chiang Mai trip, but it is eaten in other northern cities too. Unlike the northern meal laid out with sticky rice and side dishes, noodles and broth come in one bowl. It is a good choice when you are eating alone or want a light lunch.
No. It is a signature dish eaten in many northern cities. If you are meeting northern food for the first time in Chiang Mai, you could start with a bowl of khao soi.
There are several menu items, such as chicken or beef. Look at the main ingredient written on the menu and choose what you want. Not every shop uses the same meat.
If you want to read more about these dishes and regions, these sources are a good place to start. Ingredients and cooking methods can differ from one eatery to another.
Sharing image credit: Takeaway · CC BY-SA 3.0