Regions are an organized areas of the Pokémon world known from previous games about Pokémon such as games from core series. In Pokémon GO there are currently only three regions referred, which are Kanto, Johto and Hoenn.
The Pokémon known from Kanto region were the base Pokémon released in Pokémon GO. But not all Pokémon of this Generation were released at this time.
There is currently only one Pokémon that is yet to be available nor scheduled to be released and it is Mew.
- Most of the Pokémon from Kanto was released with the game release.
- November 22nd, 2016 – Ditto was released in the games, disguised as other Pokémon.
- July 22nd, 2017 to July 30th, 2017 – Articuno was available from Raid Battles.
- July 31st, 2017 to August 6th, 2017 – Moltres was available from Raid Battles.
- August 7th, 2017 to August 12th, 2017 – Zapdos was available from Raid Battles.
- From August 12th, 2017 to August 31st, 2017 - All three Legendary Birds were available from Raid Battles.
- On August 14th, 2017, during the Pokémon GO Stadium event, Mewtwo was confirmed as a Raid Boss.
Johto is so far the second region from which Pokémon are available in Pokémon GO. The Pokémon from this region were firstly released in December, 2016 as Baby Pokémon available to hatch from Pokémon Eggs.
- December 12th, 2016 - Togepi and Pichu along with another Baby Pokémon and Togetic as evolution of Togepi were released as Pokémon available only to hatch from Pokémon Eggs.
- February 16th, 2017 - over 80 Pokémon from this region became available to catch in the wild and hatch from eggs.
- From July 22nd, 2017 to August 31st, 2017 - Lugia was released as the Legendary Raid Boss and were available to catch only after defeating it.
- From August 31st, 2017 to September 30th, 2017 - Legendary Beasts were available as the Legendary Raid Bosses and were available to catch only after defeating them. All three of these Pokémon were exclusive to following regions:
- On September 30th, all three of Legendary Beasts moved to different locations and are available until October 31st in following regions:
- On October 30th, all three of Legendary Beasts moved for last time to different locations and were available until November 31st in following regions:
- From October 27th, 2017 until December 14th, 2017  - Ho-Oh was released as the Legendary Raid Boss and were available to catch only after defeating it.
- December 20th, 2017 - Delibird was announced to be released along with 20 Ice and Water-type Pokémon from Hoenn.
- It was released day later, on December 21st, 2017.
Hoenn is the third region from original core series of which Pokémon are now available in Pokémon GO. Selected Pokémon from this region were mostly released in 5 waves:
- In October 2017 as a part of Halloween event - 5 Ghost-type Pokémon.
- In December, 2017 as main release of Hoenn-region Pokémon - 50 selected Pokémon.
- In later December, 2017, more Pokémon of this region were made available - 20 aquatic themed, mainly Water and Ice-type Pokémon.
- In January, 2018, yet more were made available - 23 dessert themed, mainly Ground and Rock-type Pokémon.
- In February, 2018, more selected, sky themed, mainly Flying and Dragon-type Pokémon.
Listed waves don't include Hoenn-region Legendary Pokémon that were temporally released as Raid Bosses. Currently there are 15 Pokémon that are yet to be released.
- On October 20th, 2017, five Pokémon of Ghost and Dark types were released as a part of Halloween event. Those were Sableye, Shuppet, Banette, Duskull and Dusclops.
- December 6th, 2017 - 50 selected Pokémon from Hoenn-region were announced to become available to catch in the wild and hatch from eggs later that week along with Weather game feature. 
- From December 15th, 2017 to January 15th, 2018 - Groudon was available to be defeated and caught in Raid Battle. 
- On December 20th, 2017 - 20 Water and Ice-type Pokémon from Hoenn region were announced to catch in wild and they were released day later.
- From December 12th, 2017 to February 14th, 2018 - Kyogre was available to be defeated and caught in Raid Battle.
- On January 23rd, 2018 - 23 mostly Rock and Ground-type Pokémon from Hoenn region were announced and later this day released to catch in wild.
- On February 8th, 2018 - 19 mostly Flying and Dragon-type Pokémon from Hoenn region were announced to catch in wild and they were released day later. 
- From February 9th to March 16th, 2018 - Rayquaza is available to be defeated and caught in Raid Battle. 
- Another term used in Pokémon GO players community to describe Pokémon groups from each region is Generations. Which also is partially valid but it wasn't officially referred in game nor by its creators on official social-media and information channels.
- Therefore Kanto is Generation I, Johto is Generation II, Hoenn is Generation III and so on.
- More about generations is available here.
- Common thing for Pokémon, that were not available with main release of Kanto- and Johto-region Pokémon, is that those Pokémon either were legendary and/or mythical or have certain, special moves like Ditto, Delibird or Smeargle.
- Pokémon from Hoenn region although didn't have their main release as they were released in smaller groups.
- There has been Pokémon species from Hoenn added to game code since update 0.71.0.
- Taking into account, that new Pokémon are released in the game every 6-8 month, Pokémon from Sinnoh region might be released on the 2nd anniversary of the game.
- On December 5th, 2017 in Apple Watch Store, Pokémon GO banner with Starter Pokémon from Hoenn region appeared- spoiling official announcement about release of Hoenn-region Pokémon that happened next day. 
- Release of fifty Pokémon from Hoenn region and Weather feature had trailer with similar cinematic style as trailer for release of Legendary Pokémon in raid battles and new gyms system and trailer of the game itself.
- There are at most sixteen known regions that have appeared in the various Pokémon canons.
- ↑ Mewtwo Spotted at the Stadium!. /r/TheSilphRoad. Retrieved on 2017-08-14.
- ↑ Ho-Oh will be available to battle for an additional 48 hours Retrieved on 2017-12-06.
- ↑ Introducing Weather and 50 Pokémon from the Hoenn Region in Pokémon GO! Retrieved on 2017-12-06.
- ↑ PokemonGOApp - Trainers, dozens of Pokémon originally from the Hoenn region have been spotted all over the world! - Twitter Retrieved on 2017-12-11.
- ↑ Groudon Appearing in Raid Battles around the World! Retrieved on 2017-12-20.
- ↑ Kyogre Appearing in Raid Battles around the World! Retrieved on 2018-01-13.
- ↑ 7.0 7.1 Discover Rayquaza and More Pokémon Originally from the Hoenn Region! Retrieved on 2018-02-08.
- ↑ Gen 3 in Apple Watch store! (With minor typo) /r/TheSilphRoad. Retrieved on 2017-12-05.