Waluigi usually wears a purple undershirt under black overalls, orange shoes, and a purple cap that covers short, brown hair. He has a large, pink nose, a thin mustache which is straight (horizontal) and pointed up at the edge (forming an "L" much as Wario's forms a "W"), and gray eyes, surrounded by rings that appear to be a result of insomnia. He also has long legs which sometimes work to his advantage in sports. The yellow symbol on his hat and gloves is a vertical mirror image of Luigi's "L" which forms a "Γ", just as Wario wears a "W" in contrast to Mario's "M". The "Γ" is actually the Greek letter Gamma, which is pronounced similarly to "G" (as in go).
Just like his brother, Waluigi is the extreme form of his rival. While Luigi is tall and skinny, Waluigi is very tall and very skinny. Wario routinely puts Waluigi on a rack to make him even taller. Many biographies describing him in Mario sports games mention (and poke fun at) his odd facial elements, including his sharp jaw, pointy nose, and flat feet.
Waluigi's clothing also breaks the palette-swap color scheme in place from Mario and Luigi. Mario and Luigi have the same colored pants, with their signature colors being opposites (red and green). With Wario and Waluigi, while the signature colors are still opposites (yellow and purple), their pants are not matching colors. Waluigi was designed to wear dark purple pants, rather than the lighter purple of Wario (in order to not be monochromatic), which breaks from the previous system of color-coding. On a side note, Waluigi has appeared wearing black pants at times.
Waluigi's outfit is sometimes referenced by the Mario Bros. In Paper Mario: The Thousand-Year Door, by wearing both the "L" Emblem (Luigi) and the "W" Emblem (Wario) together, Mario will wear a purple undershirt and black overalls. The same can be done in Super Smash Bros. Brawl, by switching Luigi's costume.