This paper describes the duchamp source finder, a piece of software designed to find and describe sources in three‐dimensional, spectral‐line data cubes. duchamp has been developed with H i (neutral hydrogen) observations in mind, but is widely applicable to many types of astronomical images. It features efficient source detection and handling methods, noise suppression via smoothing or multi‐resolution wavelet reconstruction, and a range of graphical and text‐based outputs to allow the user to understand the detections. This paper details some of the key algorithms used and illustrates the effectiveness of the finder on different data sets.