Which main-series Pokémon games first introduced online trading through the Global Trade Station?

Pokémon Diamond and Pearl first introduced online trading through the Global Trade Station.

Released for the Nintendo DS, Diamond and Pearl used the system’s Nintendo Wi-Fi Connection to let players trade Pokémon online. The Global Trade Station, commonly called the GTS, allowed players to deposit a Pokémon and request another species in return, subject to the game’s trading conditions.

This was a major change from earlier Pokémon games, which required nearby systems connected by cable or local wireless features. Diamond and Pearl also supported online battles and introduced the Sinnoh region, along with 107 new Pokémon.

The original Nintendo Wi-Fi Connection service was later discontinued, so the online features of the original games no longer operate in their former official form. The GTS concept continued in later Pokémon games, including Pokémon Home and newer main-series titles, with different technical implementations.