A 12-layer hexagonal perovskite Ba₄SbMn₃O₁₂ (space group: Rm; a = 5.72733(3) Å, and c = 28.1770(3) Å) has been synthesized by high-temperature solid-state reactions and studied using powder X-ray and neutron diffraction and magnetization measurements. This 12R polytype structure contains one corner-sharing (Sb, Mn)O₆ octahedron and a trimer of face-sharing MnO₆ octahedra per formula unit. Ba₄SbMn₃O₁₂ displays a paramagnetic state of the Mn₃ magnetic cluster at 100–200 K, which partially disassociates into individual Mn ions at 250–300 K. The ferromagnetic interaction between these Mn₃ clusters is mainly mediated by Mn³⁺ at the M1 site, leading to dynamic ferromagnetic clusters below T_D = ∼70 K and ferromagnetic spin freezing transition at T_g = ∼11.5 K. The stability of Mn₃ magnetic clusters in the 12R polytypes is related to the intracluster Mn–Mn distance.