A layered sulfide, Na0.5NbS2 (space group: P63/mmc), was synthesized by a conventional solid-state reaction as an electrode material for a Na-ion battery. Galvanostatic Na insertion/extraction was performed to characterize the system NaxNbS2 (0 ≤ x ≤ 1.0) operating on the Nb(IV)/Nb(III) redox couple. Although the system shows a high specific capacity of 143.6 mAh g–1, the voltage profile is not suitable with a signature of Na/vacancy ordering at x = 0.5. First-principles calculation was applied to reveal possible structures of NaxNbS2 and describe the corresponding electrochemical properties. The calculated Na binding energies and voltages are in good agreement with experimental charge/discharge voltages. We also found a possible atomic arrangement of Na/vacancy ordering in Na0.5NbS2. Although layered NaMS2 systems allow full sodium intercalation, the strong Na+–Na+ intralayer interaction induces layer gliding and Na+-ion ordering that create undesirable steps in the voltage profile.