Cu5SbO6 is found to have a monoclinic, Delafossite-derived structure consisting of alternating layers of O–Cu(I)–O sticks and magnetic layers of Jahn–Teller distorted Cu(II)O6 octahedra in an edge sharing honeycomb arrangement with Sb(V)O6 octahedra. This yields the structural formula Cu(I)3Cu(II)2Sb(V)O6. Variants with ordered and disordered layer stacking are observed, depending on the synthesis conditions. The spin 1/2 Cu2+ ions form dimers in the honeycomb layer. The magnetic susceptibility measured between 5 and 300 K is characteristic of the presence of a singlet–triplet spin gap of 189 K. High resolution synchrotron X-ray diffraction studies indicate that changes in the intra- or interdimer distances between 300 and 20 K, such as might indicate an increase in strength of the Peierls-like distortion through the spin gap temperature, if present, are very small. A comparison to the NaFeO2-type Cu2+ honeycomb compounds Na3Cu2SbO6 and Na2Cu2TeO6 is presented.