A novel oxide AgMnO2 was prepared from LiMnO2 via Ag+ -> Li+ exchange in the eutectic melt AgNO3-KNO3. It crystallizes in a monoclinically distorted unit cell (SG C2/m) caused by the Jahn-Teller (J-T) ion Mn3+ (3d(4)). The structure was refined by isotypy with the crednerite CuMnO2. There are two long axial Mn-O of 264.2(0) pm and four equatorial bonds of 192.7(3) pin and Mn-O-Mn adjoining (83.07 degrees) are bent below the ideal angle. The thermal variation of the magnetic susceptibility (chi/T-1) obeys a Curie-Weiss law with manganese in a trivalent, high spin (HS) state accommodated in elongated MnO6 octahedra (14.8%). Direct coupling between Mn3+ involves negative exchange interactions through long-range antiparallel moments with a temperature theta(P) = -436 K and a magnetic moment of 5.26 mu B/Mn3+ slightly larger than the spin only moment. The title oxide is stable in air up to similar to 680 degrees C before it decomposes into metal silver. It displays a semi-conducting behavior with an activation energy of -0.45 eV, characteristic of a conduction by low mobility polarons between Ag+/2+ where nearly all polarons are bonded. The photoelectrochemical properties of AgMnO2 have been investigated by photocurrent technique in 1 M KOH. The cathodic photocurrent J(ph) provides unambiguous oxygen insertion (0.025 oxygen by formula unit) as required by the charge compensating mechanism. The valence band is made up of Ag-4d wave functions positioned at similar to 5.14 eV below vacuum. A comparison with CuMnO2 was also reported.