Amorphous molybdenum sulfide (MoSx) is considered a suitable alternative to Pt for catalyzing the hydrogen evolution reaction (HER) in electrochemical water splitting because of its simple and inexpensive preparation process. Herein, the template-free in situ electrodeposition of amorphous MoSx at freestanding soft interfaces is reported for the first time. The biphasic catalysis of the HER by the electrodeposited MoSx at the water-1,2-dichloroethane interface with decamethylferrocene (DMFc) was also investigated. DMFc acts as an organic sacrificial reducing agent for both (NH4)2MoS4 and protons to afford MoSx and H2, respectively. The nucleation of MoSx and the HER was examined using both four-electrode voltammetry and the biphasic reactions at the liquid-liquid interfaces. MoSx particles were collected from the interface and subjected to structural and morphological characterization. The HER kinetics was investigated using the UV-Vis absorption method with varying DMFc+ concentrations. The amount of H2 evolved and the reaction rates obtained over the in situ electrodeposited MoSx catalyst were compared with those obtained over electrodeposited metallic nanoparticles and commercial MoS2 catalysts.