Two new coordination chain compounds based on octacyanoniobate(IV) are reported: {[Mn(II)(bpy)(H2O)2]2[Nb(IV)(CN)8]·5H2O}n (1) and {[Mn(II)(bpy)(H2O)2][Mn(II)(bpy)2][Nb(IV)(CN)8]·5H2O}n (2) (bpy = 2,2'-bipyridine). Both have the topology of vertex-sharing CN-bridged squares. The Mn2Nb2 units are mutually parallel in 1 and perpendicular in 2. The antiferromagnetic intrachain interactions between the Mn(II) and Nb(IV) centers in 1 and 2, typical for Mn(II)-[Nb(IV)(CN)8] family of compounds, lead to ferrimagnetic ground state of each single chain. Both compounds show bulk ferrimagnetic ordering below their critical temperatures (Tc) of 6.4 K in 1 and 4.7 K in 2. Such a behavior is due to the ferromagnetic interchain interactions, assured by the π-π contacts between the 2,2'-bipyridine rings and by the hydrogen bonds between the terminal cyanide ligands of the [Nb(IV)(CN)8]-building blocks and coordination/crystallization water molecules. 1 and 2 are rare examples of ferrimagnets promoted by interchain ferromagnetic exchange, and the Tc of 1 is one of the highest among similar 1-D coordination polymers.