Marie Constance Sasse [Sax, Saxe, Sass] (26 January 1834 – 8 November 1907) was a Belgian operatic soprano. "Her voice was powerful, flexible, and appealing", and she was one of the leading sopranos at the Paris Opéra from 1860 to 1870. She created the roles of Elisabeth in the Paris premiere of Wagner's Tannhäuser, Sélika in the world premiere of Meyerbeer's L'Africaine, and Elisabeth de Valois in the world premiere of Verdi's Don Carlos.
Born Marie Constance Sasse in Oudenaarde, to a father who was a military band-master, she studied music at the Ghent Conservatory with François-Auguste Gevaert and in Milan with Francesco Lamperti, and made her debut in Venice as Gilda in Verdi's Rigoletto in 1852.
After the death of her father she found it necessary to work as a vocalist in the cafés of Brussels and Paris. The French soprano and vocal teacher Delphine Ugalde happened to hear her at the Café Géant in Paris. Ugalde gave her singing lessons and brought her to the attention of Léon Carvalho, who at that time was the director of the Théâtre Lyrique. Sasse made her debut at that theatre using the stage name Marie Sax on 27 September 1859, performing Rosine (Countess Almaviva) in Mozart's Les noces de Figaro. One reviewer wrote: "Mlle Sax possesses a magnificent voice, but both as a vocalist and as an actress, she is in the state of raw material—material however of undeniable quality and extraordinary aptitude, and which will undoubtedly reward the discoverer. ... It is fortunate that Mlle Sax's talents were discovered at an early stage as her voice is still fresh, and she has not been long enough in the exercise of her calling to form any vicious habits. Everything is, therefore, in her favor, and, launched in her present school, time and experience will ere long render her a valuable acquisition to the lyrical stage."