Single phase Na_0.44MnO₂ powder was synthetized by a classical solid-state reaction. Using a plastic technology, we studied the potentiometric response versus Na⁺ ions in aqueous medium. A Nernstian-like response (55mV/dec) was obtained for concentrations ranging from 1M to 2×10⁻⁴M with a quite good selectivity towards Li⁺, K⁺, Mg²⁺ and Ca²⁺. Through an ageing study of this material in aqueous solution, we clearly identified the sensing material as being the Na_0.33MnO₂ phase. Furthermore, we proved that the sensing activity of this compound is routed in its ability to reversibly insert/de-insert Na⁺ ions.