The structure of the Chalkidhiki ophiolite belt, located at the border between the Vardar Zone and the Serbomacedonian Massif of the Internal Hellenides, has been investigated by gravity and magnetic methods. New density measurements and pre-existing magnetic susceptibility measurements have been used to constrain geophysical models of subsurface structure. The northern part of the ophiolite belt, which crops out locally, is interpreted as a shallow body extending to a depth of about 2.5 km and having a width of about 10–15 km. The concealed extension of the ophiolite belt to the south, under the sediments of Toroneos Gulf, is interpreted as a massive body up to 35 km wide and 8-10 km thick. The ophiolite belt represents a major allochthonous tectonic unit overlying a continental crustal basement. Its overall subsurface geometry may be explained by the early obduction of an ophiolite slab with major thickness variations along strike, modified by later Alpine thrusting.