LAUSR

2D single‐element materials, which are pure and intrinsically homogeneous on the nanometer scale, can cut the time‐consuming material‐optimization process and circumvent the impure phase, bringing about opportunities to explore new physics and applications. Herein, for the first time, the synthesis of ultrathin cobalt single‐crystalline nanosheets with a sub‐millimeter scale via van der Waals epitaxy is demonstrated. The thickness can be as low as ≈6 nm. Theoretical calculations reveal their intrinsic ferromagnetic nature and epitaxial mechanism: that is, the synergistic effect between van der Waals interactions and surface energy minimization dominates the growth process. Cobalt nanosheets exhibit ultrahigh blocking temperatures above 710 K and in‐plane magnetic anisotropy. Electrical transport measurements further reveal that cobalt nanosheets have significant magnetoresistance (MR) effect, and can realize a unique coexistence of positive MR and negative MR under different magnetic field configurations, which can be attributed to the competition and cooperation effect among ferromagnetic interaction, orbital scattering, and electronic correlation. These results provide a valuable case for synthesizing 2D elementary metal crystals with pure phase and room‐temperature ferromagnetism and pave the way for investigating new physics and related applications in spintronics.