A new dual-band 90° coupler using coupled transmission lines is proposed in this paper. As the coupler consists of only three quarter-wave coupled-line sections, it occupies less circuit area as compared to other existing dual-band branch-line couplers. A desired dual-band operation can be achieved by proper design of the coupling level for those coupled-line sections. Moreover, the coupler's configuration contains a degree of freedom so that its design is “customizable,” allowing itself to be conveniently realized on a variety of circuit and substrate technologies. As a result, it is suitable for use in many practical applications.