As an expert in the field of Pokémon, I can tell you that Pikachu, the iconic Electric-type Pokémon, is typically portrayed as male in the games and various media. This is often indicated by the presence of a small, flat nub on its lower back, which is a characteristic of male Pikachu. However, it's important to note that Pikachu can also be female, and female Pikachu have a rounder, indented marking in the same place. The gender of a Pikachu can be determined by this physical characteristic when it is visible.

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In a quick answer, Ash's Pikachu is a male. "In 'Where No Togepi Has Gone Before!' it was confirmed that Pikachu is male. His physical appearance could not be used as proof, since gender differences were not implemented until after he had debuted in the anime."read more >>