Madame X or Portrait of Madame X is the informal title of a portrait painting by John Singer Sargent of a young socialite named Virginie Amélie Avegno Gautreau, wife of Pierre Gautreau. The model was an American expatriate who married a French banker, and became notorious in Parisian high society for her beauty and rumored infidelities. She wore lavender powder and prided herself on her appearance.

Madame X was painted not as a commission, but at the request of Sargent. It is a study in opposition. Sargent shows a woman posing in a black satin dress with jeweled straps, a dress that reveals and hides at the same time. The portrait is characterized by the pale flesh tone of the subject contrasted against a dark colored dress and background.

For Sargent, the scandal resulting from the painting's controversial reception at the Paris Salon of 1884 amounted to the failure of a strategy to build a long-term career as a portrait painter in France.

Renowned for her beauty, Gautreau represented the parisienne, a new type of Frenchwoman recognized for her sophistication. The English term 'professional beauty', referring to a woman who uses personal skills to advance to elite status, was also used to describe her. Her unconventional beauty made her an object of fascination for artists; the American painter Edward Simmons claimed that he "could not stop stalking her as one does a deer." Sargent was also impressed, and anticipated that a portrait of Gautreau would garner much attention at the upcoming Paris Salon, and increase interest in portrait commissions. He wrote to a friend: