In this letter, a dual-band ring coupler based on the folded substrate-integrated waveguide (FSIW) is proposed. The artificial structure, the composite right/left-handed (CRLH) transmission lines were implemented in the FSIW by loading the interdigital capacitors in the middle metallic layer to generate a left-handed passband region below the cutoff frequency of the ordinary waveguide TE10 mode for coupler miniaturization. The FSIW is the folded counterpart of the substrate-integrated waveguide (SIW) and thus can prevent the CRLH transmission lines from radiation loss by shielding the interdigital capacitors in the waveguide. Besides, the FSIW-based coupler is able to eliminate the electromagnetic interference (noise) with adjacent objects and lend itself to multilayer circuit integration. A ring coupler that operates at 4.3 and 7.6 GHz was developed and experimentally verified. To the best of the authors' knowledge, it is the first time that a dual-band rat-race was implemented and reported based on the CRLH-FSIW structure.