The crystal structure and magnetic properties of the compound Na₅[Mn(L-tart)₂]·12H₂O (1, L-tart = L-tartrate) have been investigated over the pressure range 0.34–3.49 GPa. The bulk modulus of 1 has been determined as 23.9(6) GPa, with a compression of the coordination spheres around the Na⁺ ions observed. 1 is therefore relatively incompressible, helping it to retain its magnetic anisotropy under pressure.