In this study, poly(γ-glutamic acid)-coated Fe3O4 magnetic nanoparticles (γ-PGA/Fe3O4 MNPs) were successfully fabricated using the co-precipitation method. Fe3O4 MNPs were also prepared for comparison. The average size and specific surface area results reveal that γ-PGA/Fe3O4 MNPs (52.4 nm, 88.41 m2·g−1) have smaller particle size and larger specific surface area than Fe3O4 MNPs (62.0 nm, 76.83 m2·g−1). The γ-PGA/Fe3O4 MNPs can remove over 99% of Cr3+, Cu2+ and Pb2+, and over 77% of Ni2+ in deionized water, much higher than γ-PGA and Fe3O4 MNPs, attributed to the larger specific surface area of γ-PGA/Fe3O4 MNPs. With the solution pH higher than 6.0, γ-PGA/Fe3O4 MNPs demonstrate better removal activity. The adsorption isotherm of γ-PGA/Fe3O4 MNPs for Cr3+ fits the Freundlich model well, with the adsorption capacity of 24.60 mg·g−1. γ-PGA/Fe3O4 MNPs are strongly attracted by permanent magnet, so it is easy to separate them completely from water. With their high efficiency for heavy metal removal and easier separation, γ-PGA/Fe3O4 MNPs have great potential applications in water treatment.