Nanostructured transition metal dichalcogenides are demonstrated to be potential catalysts to produce molecular hydrogen through electroreduction of water. Finding an efficient and cost-effective catalyst as a substitute for a platinum-based catalyst for sustainable hydrogen production is still a major issue, more so for large-scale production. Herein, we have designed dendritic ferroselite (FeSe2) hybrid nanocomposites with 2D g-C3N4 and reduced graphene oxide (rGO) nanosheets, that is, FeSe2/g-C3N4 and FeSe2/rGO as electrocatalysts for hydrogen evolution reaction (HER). Interestingly, FeSe2/rGO exhibited higher performance compared to FeSe2/g-C3N4. The highly conductive 2D FeSe2/rGO hybrid with an aligned curvy rippling surface and dendritic morphology demonstrates an onset potential of 218 mV at a current density of 10 mV/cm2 versus reversible hydrogen electrode in comparison to that of FeSe2/g-C3N4 showing an onset potential of 437 mV. The detailed density functional theory (DFT) calculations were performed to investigate the intrinsic catalytic sites and Gibbs free energy (ΔGH*) of hydrogen adsorption for the HER process. The DFT calculations displayed 0.33 V less overpotential for carbon atoms of g-C3N4 (0.97 V) compared to rGO (1.3 V). In contrast, hybrids of FeSe2/rGO (0.86 V) display lower overpotential when compared to FeSe2/g-C3N4 (1.63 V), which is in agreement with experimental results. Electrochemical impedance spectroscopy reveals lower charge transfer resistance (Rct) for FeSe2/rGO. The high hydrogen evolution activity of FeSe2/rGO is due to the electrocatalytic synergistic effect of iron diselenide and rGO, contributing to the optimum free energy for HER and improved electron mobility.