Which Pokémon generation introduced 100 new species after the original 151?

Generation II introduced 100 new Pokémon species after the original 151. The new species debuted in Pokémon Gold and Silver, released in Japan for the Game Boy Color in 1999.

The second generation expanded the Pokédex from 151 to 251 and added the Johto region. Gold and Silver also introduced major mechanics that became lasting parts of Pokémon, including Pokémon eggs, breeding, held items, a day-and-night cycle, and two new types: Dark and Steel.

Players could travel to Kanto after completing the main Johto adventure, creating an unusually large post-game area. This feature helped distinguish Gold and Silver from their predecessors and made the games feel like a continuation as well as a new journey.

Generation II is sometimes confused with the number of games in the generation or with the original Pokédex count. The precise fact is that it added 100 species, taking the cumulative total to 251. Later generations added different amounts.