The Sinner (German: Die Sünderin) is a 1951 West German romantic drama film directed by Willi Forst, starring Hildegard Knef, Gustav Fröhlich and Änne Bruck. It was shot at the Bendestorf Studios and on location in Naples, Rome, and Positano. The film's sets were designed by the art director Franz Schroedter. Upon its release, it attracted significant attention, drawing two million viewers within the first three weeks, largely due to its controversial nature. However, after it became a less contentious subject, interest waned, and in the long run, it was not a financial success.

The film represented a major shift for the director, Forst, who had previously been known for escapist works such as Operette and Vienna Blood, which avoided controversial themes and embraced romanticized settings. The Sinner departed from this tradition by adopting a realist perspective, addressing taboo subjects like prostitution and euthanasia, which challenged the moral sensitivities of post-war West German society.