What was the first main-series Pokémon game to introduce the Global Trade Station?

Pokémon Diamond and Pearl were the first main-series Pokémon games to introduce the Global Trade Station.

The Global Trade Station, commonly called the GTS, allowed players to trade Pokémon over Nintendo Wi-Fi Connection. Players could deposit a Pokémon, specify what they wanted in return, and search for trades offered by other players.

Diamond and Pearl were released for Nintendo DS in Japan in 2006 and later internationally. Their online features expanded the series beyond direct local link trading, although the original GTS required players to use the Nintendo DS wireless service.

The GTS should not be confused with ordinary Pokémon trading between nearby players. Local trading existed in the series from the first generation, while the GTS introduced a global, asynchronous marketplace-style system for requesting and exchanging Pokémon.