Designing efficient electrocatalysts with low Pt loadings for hydrogen evolution reaction (HER) is urgently required for renewable and sustainable energy conversion. Here, we report a strategy that Pt nanoparticulates are spontaneously immobilized on porous MXene/MAX monolith as HER catalysts by utilizing the redox reaction between Ti3C2Tx MXene and [PtCl4]2− in H2PtCl6 aqueous solution. By taking advantage of homogeneously distributed Pt nanoparticulates on highly electrically conductive porous Ti3C2Tx/Ti3AlC2 monolith, the as-prepared electrocatalysts show high catalytic performance for hydrogen evolution. Specifically, the binder-free electrocatalysts have Pt loadings as low as 8.9 μg/cm2, with low overpotential of 43 mV at a current density of 10 mA/cm2 and low Tafel slope that three times lower than porous Ti3C2Tx/Ti3AlC2 without Pt loading. This strategy offers a new approach to constructing ultra-low Pt-loading HER catalysts on the basis of in situ redox reaction between noble metal ions and MXenes.