Pokémon: Let's Go, Pikachu! and Let's Go, Eevee! are set in the Kanto region and include the original 151 Pokémon in addition to their respective Mega Evolved forms from Pokémon X, Y, Omega Ruby and Alpha Sapphire and their Alolan Forms from Pokémon Sun and Moon.
Let's Go, Pikachu! and Let's Go, Eevee! feature common elements of the main series, such as battling non-player character Pokémon Trainers and Gym Leaders with caught Pokémon creatures. However, when facing wild Pokémon, instead of battling them with the traditional battle system like in past games, the catching of Pokémon uses a system that is reminiscent of the mobile spin-off game Pokémon Go. By using the motion controls of the Joy-Con controller or Poké Ball Plus peripheral, players can throw berries to pacify a Pokémon or Poké Balls to attempt to capture it. The action can also be performed with a button press when the Joy-Con controllers are docked to the console or in handheld mode, but this still requires using motion controls to aim. If a player uses motion controls, the catching of Pokémon is based on the player's timing rather than accuracy. Although it is possible to miss a throw, the ball is almost guaranteed to make contact with the Pokémon. One notable difference in Let's Go, Pikachu! and Let's Go, Eevee! is that wild Pokémon show up in the overworld, rather than as random encounters in grass or in caves like in previous main series Pokémon role-playing games. To start an encounter with a wild Pokémon, the player must simply approach the Pokémon in the environment.
The games' control scheme is designed to only require one Joy-Con per player, and the games support cooperative multiplayer. If another player shakes a second Joy-Con, they can join the current player and are able to participate in battles with Pokémon Trainers and wild Pokémon encounters, allowing them to aid the catching of wild Pokémon. When playing multiplayer, Trainer battles become battles of two Pokémon against one, and in wild encounters, there is the possibility for each player to throw a Poké Ball at the same time, doubling the chances of capturing the Pokémon.
Depending on the version, players start with either a Pikachu or an Eevee, which sits on the player character's shoulder in the overworld. This mechanic is similar to the "walking Pokémon" mechanic first introduced in Pokémon Yellow, in which Pikachu followed behind the player character throughout the game. The player's partner Pokémon wiggles its tail when they are near a hidden item, and it can be dressed up for further customization. Similarly to Pokémon HeartGold and SoulSilver, players may also choose a Pokémon to follow them, and some larger Pokémon can also be ridden, a mechanic first seen in Pokémon X and Y and later expanded on in Sun and Moon.
Since Pokémon X and Y, experience points are rewarded to Pokémon not only by defeating opponent Pokémon but also by catching wild Pokémon. However, in Let's Go, Pikachu! and Let's Go, Eevee! experience points rewarded by catching wild Pokémon are affected by various multiplier bonuses depending on the timing of the throw and the technique used to throw the Poké Ball, such as performing an overarm or underarm throw. The games also introduce new items called "candies," which are used to power up a Pokémon's statistics, such as hit points (HP), Attack, Defense, Special Attack, Special Defense, and Speed. Different types of candies are awarded to players who transfer their Pokémon to Professor Oak via an in-game storage box. "Combat Power" (CP) also returns from Pokémon Go.
The evolution mechanic from previous games returns in Let's Go, Pikachu! and Let's Go, Eevee!. However, as with Pokémon Yellow, the player's starting Pikachu or Eevee cannot evolve; only other Pokémon that the player has caught, including ones of the same species as the partner Pokémon. Some features, like Pokémon breeding and HMs, are absent from the games.