Static and dynamic magnetic properties of P¯31m-phase MnSb₂O₆ have been studied by means of muon-spin relaxation (μSR), high-frequency electron-spin resonance (HF-ESR), specific heat, and magnetization studies in magnetic fields up to 25 T. The data imply onset of long-range antiferromagnetic order at T_N = 8 K and a spin-flop-like transition at B_SF ≈ 0.7–1 T. Below T_N, muon asymmetry exhibits well-defined oscillations indicating a narrow distribution of the local fields. A competing antiferromagnetic phase appearing below T_N2 = 5.3 K is evidenced by a step in the magnetization and a slight kink of the relaxation rate. Above T_N, both μSR and HF-ESR data suggest short-range spin order. HF-ESR data show that local magnetic fields persist up to at least 12T_N ≈ 100 K. Analysis of the antiferromagnetic resonance modes and the thermodynamic spin-flop field suggest zero-field splitting of Δ ≈ 18 GHz which implies small but finite magnetic anisotropy.