Rubidium guanidinate, RbCN3H4, was synthesized from guanidine and rubidium hydride, and the crystal structure was determined from powder X-ray diffraction (PXRD) data. RbCN3H4 crystallizes in the orthorhombic space group Pnma (No. 62) with four formula units per cell. The guanidinate anions are arranged in double chains running along the b axis, stacked almost perpendicularly to each other to form a three-dimensional network. The rubidium cations, coordinated by 11 N atoms, occupy the vacancies of the network in a zigzag motif along the b axis. Because the PXRD structure of the CN3 core clearly indicates the N-atom functionalities and the location of the H-atom positions, the latter spatial parameters were determined from Perdew-Burke-Ernzerhof generalized gradient approximation (GGA-PBE) density functional theory calculations. The corresponding nu(NH) stretching modes can be observed in the IR spectrum, and the volume chemistry of RbCN3H4 mirrors the efficient packing of the saltlike phase.