The title salt, K(+)·C(8)H(8)NO(5) (-) [systematic name: potassium (2R,5R,Z)-3-(2-hy-droxy-ethyl-idene)-7-oxo-4-oxa-1-aza-bicyclo-[3.2.0]heptane-2-carb-oxyl-ate], a widely used β-lactam anti-biotic, is usually chemically unstable even in the solid state owing to its tendency to be hydrolysed. In the crystal structure, the potassium cations are arranged along the a axis, forming inter-actions to the carboxyl-ate and hy-droxy groups, resulting in one-dimensional ionic columns. These columns are arranged along the b axis, connected by O-H⋯O hydrogen bonds, forming a layer in the ab plane.