Which Badge is the first one Ash earns in the Pokémon TV series?

The Boulder Badge is the first Badge Ash earns in the Pokémon TV series.

Ash receives the Boulder Badge from Brock, the Gym Leader of Pewter City, after challenging the Pewter Gym. Brock initially defeats Ash in battle, but the confrontation changes when Team Rocket attacks and Ash’s Pikachu helps protect the Gym’s Pokémon.

The Boulder Badge represents the Rock-type Gym in Pewter City and is the first of the eight Kanto Gym Badges Ash seeks before entering the Indigo Plateau Conference. Brock later leaves his Gym to travel with Ash and Misty, becoming one of the anime’s most recognizable companions.

The Cascade Badge is associated with Misty’s Cerulean Gym, while the Thunder Badge belongs to Lt. Surge’s Vermilion Gym. Those badges are also earned during Ash’s Kanto journey, but neither comes before the Boulder Badge.