rGO-MoSe2 nanocomposites were prepared via a one-pot hydrothermal method in which MoSe2 microspheres (MS) were decorated on rGO sheets. Three nanocomposites named F1, F2, and F3 were prepared using different weight ratios of MoSe2 MS to rGO: (3 : 1), (4 : 1), and (5 : 1), respectively. FESEM images showed a flower-like porous morphology of the MoSe2 microspheres. All the rGO-MoSe2 nanocomposites exhibited remarkable microwave absorption properties as demonstrated by strong reflection loss (-58 dB to -99 dB) and an ultrabroad effective absorption bandwidth (equivalent to 90% attenuation), which covers whole X and Ku frequency bands at matching thicknesses of 2.8-3.2 mm. The minimum reflection loss reached -98, -99, and -75 dB for F1, F2 and F3, respectively. The excellent absorption properties of the rGO-MoSe2 nanocomposites is related to the unique morphology and micro size of MoSe2 in which incident waves are attenuated by multiple reflections and scattering.