Hybrid sodium ion capacitors (HSICs) can combine the merits of both high-energy density sodium ion batteries (SIBs) and high-power supercapacitors. Currently one of the main challenges in developing high-performance HSICs is the lack of suitable electrode material with superior Na-ion storage capability. In this work, a novel nanocomposite comprised of MoSe2 nanosheets decorated with SnO2 nanoparticles through interfacial Se-O bonding (denoted as O-MoSe2/SnO2) has been rationally synthesized and studied as an electrode for both SIBs and HSICs. The nanocomposite delivers an impressive Na-ion storage capacity of 249 mA h g(-1) even at a high current density of 10 A g(-1). Kinetics analyses using cyclic voltammetry technique reveal the Na+-ion storage in the nanocomposite is governed by a pseudocapacitive charge storage (accounting for similar to 88% at a scan rate of 1.0 mV s(-1)) with fast Na+ insertion/extraction kinetics. Density Functional Theory (DFT) calculations disclose that a charge accumulation occurs at the interface of O-MoSe2/SnO2 nanocomposite, which promotes rapid Na+-ion transport through the interface. Furthermore, a HSIC device is assembled using the O-MoSe2/SnO2 nanocomposite as anode and an activated carbon as cathode, demonstrating a high energy density of 70 Wh kg(-1) at a power-output of 62 W kg(-1) with an excellent cycling stability of high capacitance retention rate of 94% for 6000 cycles at 5 A g(-1).