The Vermilion Gym is the official Gym of Vermilion City. It is based on Electric-type Pokémon. The Gym Leader is Natalia. Trainers who defeat her receive the Thunder Badge.

The main room is filled with 15 trash cans, and Trainers are scattered about this room. Beyond this room is another, guarded by electric doors. The only way to get in the final room, where Natalia is located, is to find two switches underneath the trash cans. If the first switch was found, but the player incorrectly guessed the second one, the traps are reset. The second switch is located near the first.