Quasi-one-dimensional materials exhibit not only unique crystal structure but also abundant physical properties such as charge density wave, Luttinger liquid, and superconductivity. Here we report the discovery, structure, and physical properties of a new manganese-based quasi-one-dimensional material RbMn6Bi5, which crystallizes in a monoclinic space group C2/m (No. 12) with lattice parameters a = 23.286(5) Å, b = 4.6215(9) Å, c = 13.631(3) Å, and β = 125.00(3)°. The structure features [Mn6Bi5]-1 double-walled column extending along the [010] direction, together with Bi-Bi homoatomic bonds linking the columns and the countercation Rb+. The temperature-dependent resistivity clearly indicates a significant resistivity anisotropy for RbMn6Bi5, whereas the magnetic susceptibility and specific heat measurements show that RbMn6Bi5 is antiferromagnetic below 82 K. The density functional theory calculations indicate that RbMn6Bi5 is a quasi-one-dimensional metal with possible helical antiferromagnetic configuration. The discovery of RbMn6Bi5 confirms the viability of discovering new quasi-one-dimensional materials in manganese-based compounds.