Which Pokémon TCG expansion first introduced Dark Pokémon?

Team Rocket was the first Pokémon TCG expansion to introduce Dark Pokémon.

Released in English on April 24, 2000, Team Rocket was the fifth major English Pokémon TCG set. Its theme came from the criminal organization Team Rocket, whose members and Pokémon were prominent in the Pokémon video games and anime.

The set introduced Dark Pokémon as creatures corrupted or controlled by Team Rocket. These cards used the “Dark” designation in their names, creating a distinct mechanical and thematic identity within the early trading-card era. The set also included cards featuring the animated trio Jessie, James, and Meowth.

Team Rocket followed Base Set, Jungle, Fossil, and Base Set 2. Gym Heroes came later and focused on Gym Leaders and their Pokémon, while Fossil is remembered for the TCG’s first Ditto card. Because those sets also belong to the early English card era, they are common alternatives in this question.