Highly efficient electrocatalysts are essential for the production of green hydrogen from water electrolysis. Herein, a metal‐organic framework‐assisted pyrolysis‐replacement‐reorganization approach is developed to obtain ultrafine Pt‐Co alloy nanoparticles (sub‐10 nm) attached on the inner and outer shells of porous nitrogen‐doped carbon nanotubes (NCNT) with closed ends. During the thermal reorganization, the migration of Pt‐Co nano‐alloys to both surfaces ensures the maximized exposure of active sites while maintaining the robust attachment to the porous carbon matrix. Density functional theory calculations suggest a nearly thermodynamically‐neutral free energy of adsorption for hydrogen intermediates and diversified active sites induced by alloying, thus resulting in a great promotion in intrinsic activity towards the hydrogen evolution reaction (HER). Benefiting from the delicate structural design and compositional modulation, the optimized Pt3Co@NCNT electrocatalyst manifests outstanding HER activity and superior stability in both acidic and alkaline media. Ultrafine Pt‐Co alloy nanoparticles confined on the inner and outer surfaces of porous nitrogen‐doped carbon nanotubes (Pt‐Co@NCNT) have been synthesized through a metal–organic framework (MOF)‐assisted pyrolysis‐replacement‐reorganization approach. With the structural and compositional merits, the Pt3Co@NCNT electrocatalyst presents superior activity and stability in both acidic and alkaline media for the hydrogen evolution reaction.