A method of inserting a watermark in digital data (I) includes the steps of multi-resolution spectral breakdown (E1) of the digital data (I); extraction (E2) of the components of lowest frequency; choice (E3) of a subset of the components with the lowest frequency; modulation (E4) of the components of said subset in order to insert the supplementary information (S); and reverse multi-resolution spectral recomposition (E5) of the watermarked digital data (I).