Undyne is an anthropomorphic fish monster. She has blue scales and a long red ponytail. She has red and blue fins on the sides of her head, and a pair of sharp, yellow, protruding teeth. 

She wears a red eyeshadow and has an eyepatch over her left eye. She also wears a black tank top and blue jeans, but debuts in armor varying in light and dark shades of gray. Her eyes have black vertical pupils and yellow sclera. Undyne has no nose.