Ethel Philip, Whistler's sister-in-law and a favorite subject of the artist, whose likeness appears in drawings, etchings, lithographs, and at least five full-size oil paintings, stands against an empty, shallow background with her back to the viewer. She wears a grey silk gown topped with a bolero jacket of transparent silk gauze or fine net—a design inspired by traditional costumes from Spain's Andalusian region. Ethel's head and shoulders are turned slightly to the right to reveal her profile. This graceful pose emphasizes her costume's long, sweeping cascade to the floor, drawing the viewer's attention not to the model's features but to her dramatic attire. One art historian concluded that "it is not a portrait of a person, but of a dress," transforming Ethel Philip into a fashion model or mannequin rather than the subject of a formal, full-length portrait.
The work's title may also have sartorial origins—the result of Whistler's extended stay in Paris, a city renowned for its luxury fashion houses and couturiers. At that time, couture dresses were often given individual names to emphasize their uniqueness. It is possible that Whistler's title refers not to a Spanish woman, but to the model of the dress worn by Ethel.
Whistler had demonstrated a strong interest in women's fashion and its role in his portraiture long before he painted The Andalusian. In the 1870s, apparently dissatisfied with British fashions at the time, the artist himself designed the elaborate dresses worn by the female subjects of Harmony in Grey and Green: Miss Cicely Alexander (Tate Gallery, London) and Symphony in Flesh Color and Pink: Portrait of Mrs. Frances Leyland (Frick Collection).
More information on this painting can be found in the Gallery publication American Paintings of the Nineteenth Century, Part II, pages 248-252, which is available as a free PDF (21MB).