A series of novel magnetic branched polyethylenimine nanogels (Fe3O4/bPEI) was prepared via the combination of crosslinking branched polyethylenimine to form a nanogel and in-situ generation of Fe3O4 on its surface. Morphologies and magnetic properties of the Fe3O4/bPEI nanocomposites could be easily controlled by regulating the ratio between branched polyethylenimine nanogel and Fe3O4 precursor. The nanocomposites could be applied as efficient and selective dye adsorbents for the removal of Congo red under different pH values or in the presence of methylene blue. Furthermore, it could also be applied as catalyst support for the loading of palladium to afford a novel Pd-Fe3O4/bPEI nanocomposite, which exhibited good catalytic activity in the reduction of nitrophenols using NaBH4 as the reducing agent. Particularly, this nanocomposite could be easily separated by an external magnetic field and recycled ten times without appreciable loss of its initial catalytic activity. The synergistic integration of nanogel and magnetic Fe3O4 nanoparticles makes Fe3O4/bPEI to be a versatile platform for multiple applications.