The invention relates to a method of detecting a watermark W, formed by a set of coefficients, in an information signal S, said watermark W and said information signal S being configured in the form of matrices SM M*M. The coefficients of the Fourier transform matrix of the matrix of the signal TSM and the coefficients of the Fourier transform matrix of the watermark matrix TWM are used in a calculation step CAL of the inverse transform matrix of the multiplication ITM of the transform matrix of the signal TSM and of the transform matrix of the watermark TWM, said calculation CAL being produced in sequences which each result in obtaining I columns of the inverse transform matrix of the multiplication, where M is a multiple of I and different from I. Said I columns are stored in a memory called internal memory and the detection step DET of the watermark is achieved with each sequence by detection of peaks of each set of the I columns while said I columns are present in the internal memory. The invention permits the use of a small-size internal memory.