I Was a Spy is a 1933 British thriller film directed by Victor Saville and produced by Michael Balcon. It stars Madeleine Carroll as Marthe Cnockaert, Herbert Marshall, and Conrad Veidt. Based on the 1932 memoir I Was a Spy by Marthe Cnockaert, the film is about her experiences as a Belgian woman who nursed German soldiers during World War I while passing intelligence to the British. The film was produced by Gaumont British Picture Corporation with Woolf & Freedman Film Service and Fox Film Corporation distributing in the United Kingdom and the United States respectively.

I Was a Spy was also the first film dubbed in Poland (while there were earlier examples of films dubbed in Polish, they were recorded in Paramount studio in Joinville, France), released in 1935 as Siostra Marta jest szpiegiem, starring Lidia Wysocka as Martha Cnockhaert's voice. The screenplay was written by Ian Hay, W. P. Lipscomb and Edmund Gwenn.

I was a Spy was released to cinemas in the United Kingdom on 4 September 1933. It was voted the best British film of 1933 and the performance of Carroll was praised.