Hydrogen evolution reaction (HER) has prospect to becoming clean and renewable technology for hydrogen production and Ni–Mo alloy is among the best HER catalysts in alkaline electrolytes. Here, an in situ topotactic reduction method to synthesize ultrathin 2D Ni–Mo alloy nanosheets for electrocatalytic hydrogen evolution is reported. Due to its ultrathin structure and tailored composition, the as‐synthesized Ni–Mo alloy shows an overpotential of 35 mV to reach a current density of 10 mA cm⁻², along with a Tafel slope of 45 mV decade⁻¹, demonstrating a comparable intrinsic activity to state‐of‐art commercial Pt/C catalyst. Besides, the vertically aligned assemble structure of the 2D NiMo nanosheets on conductive substrate makes the electrode “superaerophobic,” thus leading to much faster bubble releasing during HER process and therefore shows faster mass transfer behavior at high current density as compared with drop drying Pt/C catalyst on the same substrate. Such in situ topotactic conversion finds a way to design and fabricate low‐cost, earth‐abundant non‐noble metal based ultrathin 2D nanostructures for electrocatalytic issues.