We propose and demonstrate an all‐optical correlator based on modal dispersion in a multimode fiber. Many of the modes can be excited in a large core high numerical aperture step‐index multimode fiber under overfilled launching. The incident optical signal is copied into different modes and a time delay is introduced by the modal dispersion. We use a mask with slits as a space filter to select the modes needed. A correlator of the mask and input signal is achieved at the output end. Thanks to the use of modal dispersion, the correlator is nearly independent of the wavelength and bandwidth of the input signal. By adjusting the slits on the mask, the target patterns of the correlator can be changed easily. A radio frequency signal detection is also experimentally demonstrated with this construction.