Pokémon Let's Go Pikachu & Eevee, unlike most mainline series games, have got some connectivity with spin-off titles. In this case, it will connect to Pokémon GO. There are two facets to this.

Pokémon GO to Pokémon Let's Go

The most notable bit of connectivity goes from Pokémon GO to Pokémon Let's Go. With this, you can send Pokémon from the Kanto Pokédex that you caught in Pokémon GO into a special area in Let's Go known as "GO Park". This area has you interact with the various Pokémon on the overworld and then capture them.

Only Pokémon from Bulbasaur to Mewtwo can be sent through this, including the Alola Forms of Pokémon with Alola Forms. Mew cannot be transferred.

This capture is only one way. Once you have caught it in the GO Park, it cannot return to Pokémon GO. When you transfer to Pokémon Let's Go, the Pokémon GO game gets one candy for each Pokémon. IVs are calculated based on the Pokémon's IVs in Pokémon GO

GO IV Let's Go IVs Calculation
StaminaHit Points*2 + 1
Special Attack
*2 + 1
Special Defense
*2 + 1

For full details on GO Park, check our GO Park Section.

Pokémon Let's Go to Pokémon GO

There is some connectivity with Pokémon Let's Go into Pokémon GO. Through the game, you'll have the ability to send a special Mystery Box to Pokémon GO. To do this, you have to connect your Pokémon GO game to a Pokémon Let's Go game and you'll automatically be sent the Mystery Box. When it is open, for 30 minutes you will have the opportunity to capture up to 10 Meltan. Meltan can also be sent to Let's Go

This box can be reopened after a week once another Pokémon has been sent to Pokémon Let's Go. After this time, to reopen it, you need to send a Pokémon back to a Pokémon Let's Go game.

This connectivity is required to get more than 1 Meltan and Melmetal in Pokémon Let's Go.

Connecting Your Switch & Pokémon GO

Syncing Pokémon GO & Pokémon Let's Go

To connect Pokémon GO with Pokémon Let's Go, you need Pokémon GO to be Version 1.97.1/0.127.1 or higher. Here, you can go into the Settings Menu and you'll find a Nintendo Switch option. Click that and tap the "Connect to Nintendo Switch" link.
On the Nintendo Switch, open the main menu with the X or B button and press Y or shake the controller to go to the Options. There, you can find a Pokémon GO Settings option where you then click through and the games will find eachother through Bluetooth

Pokémon GOPokémon Let's Go
Sending Pokémon from Pokémon GO to Pokémon Let's Go

To send a Pokémon, you need to go into the Pokémon menu. If your Pokémon GO has connected to a Nintendo Switch, then there will be a Nintendo Switch icon. If you tap that, your menu will shift to Pokémon that you can send, blocking out Pokémon you have favourited, Pokémon not in the Kanto Pokédex and event Pikachu, Squirtle, Wartortle & Blastoise with unique hats/clothing. You can select up to 30 Pokémon at any one time. This is entirely one way as the Pokémon cannot return to GO.
In Pokémon Let's Go, speak to the receptionist in the GO Park and they'll offer the option to Bring in Pokémon. Select which GO Park of the 20 you wish to send them to and they'll connect to Pokémon GO and the process will begin

Pokémon GOPokémon Let's Go

After sending the Pokémon, they can then be accesed in the Pokémon GO Park in Pokémon Let's Go, Pikachu! & Let's Go, Eevee! and Pokémon GO will notify you of the Mystery Box, saying if it's ready to open or showing how long until it can open again.