Black phosphorus (BP), a promising two-dimensional material, faces significant challenges for its applications due to its instability in air and water. Herein, molecular dynamics simulations reveal that a self-assembled ferrocene (FeCp2) molecular layer can form on BP surfaces and remain stable in aqueous environments, predicting its effectiveness for passivation. This theoretical finding is corroborated by X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, Raman spectroscopy, and optical microscopy observations. In addition, atomic force microscopy analysis confirms that ferrocene-passivated BP flakes with thicknesses of <10 nm exhibit minimal degradation over 25 days. Density functional theory calculations further show that ferrocene stabilizes BP and modulates its band gap, improving its electronic applicability. Notably, we find that the passivation of BP by metallocenes is universal because other metallocenes (VCp2, MnCp2, and NiCp2) exhibit similar adsorption behaviors. These findings underscore the potential of metallocenes as versatile protective layers for BP and other materials that are not stable in air.