A Fe3O4/activated montmorillonite (Fe3O4/Mt) nanocomposite was prepared by a coprecipitation method. The morphology and structure of the Fe3O4/Mt nanocomposite are explored using X-ray powder diffraction, Fourier transform infrared spectroscopy, N2 adsorption–desorption isotherm and scanning electron microscopy. The adsorption capacity of Fe3O4/Mt for methylene blue (MB) was evaluated. Within 25 min, the Fe3O4/Mt nanocomposite (0.5 g) removed 99.47% of the MB from a 120 mg L− 1 solution at 293 K and at an original solution pH of 7.37. The experimental adsorption data followed a pseudo-second-order kinetic model and Langmuir isotherm. The reusability of the Fe3O4/Mt was tested and still over 83.73% color removal of MB was obtained after five cycles. The Fe3O4/Mt nanocomposite has good stability and reusability.