In which countries is Pokemon Go available?

Pokemon Go is out in Australia, New Zealand, Japan, and is slowly rolling out across the United States and other countries. Just be patient and regularly check your app store/Google play.

Be warned anywhere boasting to get you the game now(before it is available normally for you) could lead to you potentially downloading Malware on your phone or (less likely but still a possiblity) being banned from ever playing the game on your device. So just be patient, the game should be worldwide soon.

Unofficial Source

Pokémon GO - Get up and go!Break out the sneakers and Poké Balls! Pokémon GO is now available in the US, Australia and New Zealand on iPhone and Android devices. Coming to other countries soon!

Official Source

Update: Germany and UK also seems to be out now.

Update: Italy and The Netherlands should be out now too. Check this website for future updates: https://www.ispokemongoavailableyet.com/

From Niantic's announcement page

We are pleased to announce that Pokémon GO, the next evolution of Real World Gaming, is now officially available on both the App Store and on Google Play Store in Australia, New Zealand and the United States. It will be available in other countries around the world in the days ahead.

But this announcement doesn't explain if it's only available to install and play in those countries or if you can install the app in those countries and travel abroad and be able to play in different countries, too. This question has to be answered by the community. There isn't any official statement about it, yet.

As per Bulbapedia, this is the list of the countries where Pokémon GO is available.

The freemium mobile game Pokémon GO is now available for iOS and Android in 26 more European countries.
Specifically, it is now available in Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, Greece, Greenland, Hungary, Iceland, Ireland, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Romania, Slovakia, Slovenia, Sweden, and Switzerland.

This follows its release last week in Australia, New Zealand, and the United States; its release three days ago in Germany; and its release two days ago in the United Kingdom; and its release yesterday in Italy, Spain, and Portugal. Notably, it is still not available in France, Turkey or Russia.

As previously reported, the game is stated to be coming soon to Japan, Canada, and Latin and South America.

Also, you can check the avalability from this website: http://www.releases.com/