Spinel-type lithium antimony manganese oxide was first synthesized by aging the precipitates that were obtained by reaction of a mixed aqueous solution of manganese(II) and antimony(V) chlorides (Sb/Mn = 0.25) with (LiOH + H2O2) solution, followed by hydrothermal treatment at 120 °C. A well crystallized spinel-type solid with a chemical composition of Li1.16Sb(V)0.29Mn(III)0.77Mn(IV)0.77O4 was obtained; it had the characteristic of including mixed valence manganese. The spinel structure was preserved during heating up to 600 °C. Most of the lithium ions in the prepared material could be extracted by treating with an acid; the extraction progressed topotactically, preserving the spinel structure, accompanied by a decrease in the lattice constant from 8.31 ± 0.01 Å to 8.13 ± 0.01 Å. The mean oxidation of manganese increased from 3.50 to 3.98 after acid treatment, due to the disproportionation of trivalent manganese. The DTA-TG analysis and IR spectra showed the formation of lattice protons exchangeable with lithium ions. The pH titration study showed that the acid treated sample had a remarkable lithium ion-sieve property over the entire pH range studied. The affinity order was K < Na ≪ Li and the exchange capacity reached 5.6 mmol g−1 for Li+. It showed a selective uptake of 14 mg g−1 of Li+ from LiCl enriched seawater.