The crystal structure of a new form of dehydrated mang­a­nese(II) acetate, poly[[hexa-[mu]3-acetato-trimanganese(II)] aceto­nitrile solvate], [Mn3(CH3COO)6]·CH3CNn, (I), reveals a three-dimensional polymeric structure based on an Mn3 trimer. The Mn3 asymmetric unit contains three crystallographically independent Mn positions, comprising a seven-coordinate center sharing a mirror plane with a six-coordinate center, and another six-coordinate atom located on an inversion center. Two of the four crystallographically independent acetate (OAc) ligands, as well as the acetonitrile solvent mol­ecule, are also located on the mirror plane. The Mn atoms are connected by a mixture of Mn-O-Mn and Mn-OCO-Mn bridging modes, giving rise to face- and corner-sharing inter­actions between manganese polyhedra within the trimers, and edge- and corner-sharing connections between the trimers. The network contains substantial pores which are tightly filled by crystallographically located acetonitrile mol­ecules. This structure represents the first porous structurally characterized phase of anhydrous manganese(II) acetate and as such it is compared with the closely related densely packed anhydrous manganese(II) acetate phase, solvent-free [beta]-Mn(OAc)2.