The new ternary vanadate Na₃MnV₂O_7.5 has been synthesized by solid state reaction route. Its crystal structure was determined from both powder X-ray diffraction data and high-resolution transmission electron microscopy (HRTEM) analysis. Na₃MnV₂O_7.5 (P2/c, a = 7.4004(4) Å, b = 5.6065(3) Å, c = 17.0876(11) Å, β = 90.0114(2) °, and Z = 4) crystallizes with a new type of oxygen-deficient perovskite ABO_3-δ (δ = 1/2) with the sodium atoms occupying the A site and the vanadium and manganese atoms perfectly ordered in the B site. In Na₃MnV₂O_7.5 the manganese and the vanadium atoms are octahedral and tetrahedral coordinated, respectively. The MnO₆ and VO₄ polyhedra share edges and form a perovskite 3D-framework with high disorder of the oxygen atoms. HRTEM confirmed this structural model. Na₃MnV₂O_7.5 was characterized by magnetic susceptibility and specific heat measurements. It is antiferromagnetic with no magnetic ordering down to 2 K.