Teams

Teams are in-game groups that Trainers can join, led by NPC Team Leaders. Trainers must be at least level 5 to join a team by tapping any Gym. A trainer's team is displayed in their profile.

Since February, 2019, Trainers are able to change their team, once a year, after purchasing for and activating a Team Medallion.

Gameplay
Team members can work together to defeat and claim enemy Gyms or help defend friendly Gyms. A trainer can spin a friendly Gym's Photo Disc to earn bonus items. Team members can also work together to earn bonus Premier Balls during Raid Battles, and the team that controls the Gym during the Raid will also earn bonus Premier Balls.

When a trainer joins a team, that team's leader will also provide Pokémon Appraisal. The information is the same between all three teams, but each team leader has different mannerisms when appraising a Pokémon.

Team affiliation does not have any other in-game effects. There are no team-exclusive Pokémon, and the mascot Pokémon do not have any extra effects for team members.

Descriptions
Each Team holds certain values, beliefs, and goals.

Team Valor relies on strength in battle. Valor's members believe that Pokémon are stronger and more warmhearted than humans and are interested in enhancing their natural power.

Team Mystic relies on analyzing every situation. Mystic's members believe that Pokémon have immeasurable wisdom and are interested in learning more about why Pokémon experience evolution.

Team Instinct relies on a trainer's instincts. Instinct's members believe that Pokémon have excellent intuition and are interested in learning more about its connection to the egg hatching process.

Trivia

 * Prior to version 0.135.0 and since the game's inception, Trainers were unable to change their team after choosing one.
 * It is possible for a player to not join any team.
 * Instead of one of the Legendary Birds, their profile would have a green logo of a Poké Ball as a background. Any part of the interface with a team-specific colour (e.g. experience bar, buddy candy progress) would also be green, instead of red, blue, or yellow.
 * They are unable to participate in Gym or Raid Battles (and thus cannot earn PokéCoins without paying real money; if they ever tap a gym, they must close the game and restart it to avoid being forced to pick a team), but they can fight in Trainer Battles, including Team Leader battles and Team GO Rocket Grunts battles. In the latter case, the three Team Leaders would randomly appear to give advice regardless.
 * They can battle Team GO Rocket Leaders, and similarly a random friendly Team Leader would speak if the Rocket Radar cannot find any Rocket Leader, or advice them if they are defeated by a Rocket Leader. They cannot fight Giovanni however as Looming in the Shadows requires winning raid battles.
 * GO Fest Weekly Challenge: Battle notably gives a Super Rocket Radar without this requirement, marking the first time a player without a team can fight him. Suicune is thus the first Shadow Lengendary obtainable by those players.
 * The ticketed Special Research during Pokémon GO Fest 2020 allows encounters with Shadow Articuno, Moltres, Zapdos, and Mewtwo.
 * They cannot complete certain Research tasks, including those requiring you to fight in gym or raid battles. This also prevents them from obtaining any Mythical Pokémon from Special Research, as obtaining Meltan or Mew both require gym and raid battles on the third part of their task sets, and Mew's task set (A Mythical Discovery) must be completed up to the third part to unlock other Mythical task sets. However, obtaining Meltan is still possible by using Mystery Box after connecting game account to Pokémon Let's Go game, and the aforementioned Pokémon GO Fest 2020 research allows an encounter with Victini.
 * They are unable to use the Pokémon Appraisal feature, although they can still use Pokémon search to find Pokémon of a certain broad rating.
 * Mega Evolution is limited, as they cannot do raids.