Pt-doping of an ultrathin Pd nanosheet results in the unprecedented structural rearrangement of a Pd nanosheet into a PdPt nanotent structure, in which a tripod stands on a triangular nanosheet. Further growth of Pt phase on this nanotent structure is dependent on the presence of surface-stabilizing CO molecules, leading to the formation of two distinct nanoscale phase segregated structures with respective structural features of a popped out Pt facet and an overgrown Pt layer.