The Mn ²⁺ -containing oxide Mn ₃ Ta ₂ O ₈ crystallises in the space group I4 ₁ /a with a=11.2728(2) and c=9.8030(3) Å, and with a structure related to the cubic fluorite type. The compound is antiferromagnetic with a Néel temperature of 23 K. The magnetic structure at 10 K was determined from constant-wavelength (λ=1.47 Å) neutron powder diffraction (NPD) data and refined by the Rietveld method to R _F =2.1% and S=1.3 for 80 reflections with magnetic contributions at 2θ≤60°. The magnetic moments, of magnitude 4.01(2) µB, lie in the ab plane and are antiparallel with the (001) planes. The spin ordering is similar to that in the cubic fluorite structure of UO ₂ but with 40% of the magnetic atoms replaced with non-magnetic Ta atoms. The non-magnetic crystal structure at 295 K was refined to R _F =0.6% using time-of-flight (TOF) NPD data, anisotropic thermal parameters and 1943 reflections with d≥0.55 Å.