Which Pokémon game first introduced online multiplayer trading and battling in the main series?

Pokémon Diamond and Pearl were the first main-series games to introduce online multiplayer trading and battling.

Released for Nintendo DS, Diamond and Pearl used the Nintendo Wi-Fi Connection to let players connect over the internet. Earlier games supported trading and battling locally, usually through a link cable or nearby wireless connection.

The games also added the Global Trade Station, which allowed players to search for requested Pokémon and offer creatures for exchange. This expanded the series’ central trading concept from a local activity into a worldwide system.

Pokémon Platinum later expanded the same fourth-generation setting and features. Pokémon HOME is a much newer service and should not be mistaken for the original online multiplayer system introduced in Diamond and Pearl.