The development of efficient and cost-effective electrocatalysts for the hydrogen evolution reaction (HER) is crucial for achieving sustainable hydrogen production. Molybdenum phosphide (MoP) represents a promising non-noble-metal catalyst whose HER overpotential is higher than that of Pt. In this study, Pt-loaded MoP dispersed on carbon nanotubes (Pt–MoP/CNTs) is fabricated via an oxalate-assisted molecular self-assembly route, followed by phosphorization and low-temperature Pt incorporation. The introduction of an ultralow Pt loading (2 wt%) significantly enhanced the HER performance in acidic media. At a current density of 20 mA cm⁻², the Pt–MoP/CNTs required an overpotential of 70 mV, which is markedly lower than that required by pristine MoP/CNTs (180 mV). The Pt-modified catalyst also exhibited a high double-layer capacitance of 532 s⁻¹ at an overpotential of 28 mV, together with a large electrochemically active surface area, improved charge-transfer kinetics, and excellent stability over 48 h of continuous operation. The synergistic interaction between Pt and MoP effectively boosted catalytic activity while minimizing the use of noble metals. This work demonstrates an efficient strategy for the design of high-performance and economically viable HER electrocatalysts through trace-platinum modification of transition-metal phosphides.