This study presents a novel ultrathin dual-band frequency selective surface (FSS) for X-band applications. The proposed single-layer FSS operates as a band reject filter at 8.47 and 10.45 GHz. The ratio of these two operating bands has small value such as 1.23, and it points out that the proposed FSS has closely spaced characteristic. In addition, the designed dual-band single-layer FSS has an ultrathin thickness of 0.021*lambda(l), where lambda(l) is the wavelength of the lower operational frequency. The stable frequency responses are sufficiently achieved for the transverse electric and transverse magnetic polarizations by using the proposed FSS. The validity of FSS is obtained by comparing measurements and simulations.