A device for transcoding a video data stream in the H.264 format into a video data stream in the H.265 format includes circuitry adapted for performing a merging of adjacent macroblocks of the data stream in the H.264 format according to a predetermined criterion of frequency similarity of the discrete cosine transform coefficients of said macroblocks after application of an inverse quantisation operation; and for determining motion vectors in the H.265 format on the basis of motion vectors in the H.264 format by performing, for the macroblocks resulting from the merging of macroblocks issuing from the data stream in the H.264 format, a combination of the motion vectors of said macroblocks issuing from the data stream in the H.264 format.