The first structural characterization of the transient triply-reduced state of corannulene (C20H10) is accomplished. The X-ray crystallographic study reveals that the C20H10c3- trianions, generated by corannulene reduction with metallic cesium, form a novel type of supramolecular sandwich-type assembly, [Cs+//(C20H103-)/4Cs+/(C20H103-)//Cs+]. In the product, two triply-charged corannulene decksencapsulate a rectangle of four cesium ions with the external concave bowl cavities being filled by one cesium ion each. The structural investigation is augmented by in-depth theoretical calculations to provide insights into the geometrical features and electronic structure of this unique organometallic self-assembly.