In typical rare-earth lanthanide compounds, the localized $4f$ electrons have a weak effect on the electrical conduction, limiting their influence on the Berry curvature and, hence, the intrinsic anomalous Hall effect. A comprehensive study of the magnetic, thermodynamic, and transport properties of single-crystalline NdGaSi, guided by first‐principles calculations, reveals a ferromagnetic ground state that induces a splitting of quasiflat $4f$ electronic bands and positions them near the Fermi energy. The observation of an extraordinarily large intrinsic anomalous Hall conductivity of $1165{\mathrm{\Omega }}^{-1}{\mathrm{cm}}^{-1}$ implies the direct involvement of localized states in the generation of nontrivial band crossings around the Fermi energy. The angle-resolved photoemission spectroscopy measurements provide direct evidence of nontrivial crossing of the $4f$ bands with dispersive bands. These results are remarkable when compared to ferrimagnetic NdAlSi, which differs only in a nonmagnetic atom (a change in the principal quantum number $n$ of the outer $p$ orbital) with the same number of valence electrons and does not exhibit any measurable anomalous Hall conductivity.

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• Electrical conductivity
• First-principles calculations
• Flat bands
• Hall effect
• Magnetotransport
• Spin-orbit coupling
• Topological materials
• Heavy-fermion systems