If I buy Apple Watch with cellular in USA, will it also work in India for cellular?

Hello,

There is an explanation in this article about roaming with your Watch.

You can read about it under the heading 'About international roaming'.

Set up cellular on Apple Watch - Apple Support

Have a great day.

A cellular watch bought in the US will only function in North America,Canada,Mexico and Puerto Rico anywhere else it will function as a GPS only watch.