Nathalie "Lily" Sergueiew (January 24, 1912 – May 17, 1950) was a double agent who worked for MI5 during World War II under the codename "Treasure".

Her Abwehr case officer, Major Emil Kliemann, trained her in intelligence gathering and communications techniques, and in 1943 sent her to Spain, along with her beloved dog “Babs”.

After a tumultuous meeting with Colonel T. A. Robertson, head of the section responsible for control of the Double Cross agents, she eventually revealed the secret code.

Sergueiew joined the Free French forces in England; after training, she returned to France as a liaison officer, working with Displaced Persons.

Her last posting was to liaise with the US army officer in charge of the Ehrfurt area, including the infamous Buchenwald camp, Major John Barton ("Bart") Collings.

These memoirs (with critical comments on her former MI5 employers) were eventually found by Gilles Perrault and published in 1966, entitled Seule face à l'Abwehr ["Alone against the Abwehr"], (Paris: Librairie Arthème Fayard / Éditions J'ai Lu).