We present aNyquist-filtering optical (de)multiplexer using a ring resonator-assisted interferometer circuit. It features a near-rectangular passband spectrum, scalable port count, and can be designed with sub-GHz spectral resolution. The circuit can be constructed using ordinary passive photonic-integrated circuit building blocks. In contrast to its counterparts using tapped-delayline circuit topologies, this circuit has a chip area two-orders-ofmagnitude smaller. The results of this study show the potential for realizing high-spectral-efficiency multicarrier transceivers and reconfigurable optical add-drop multiplexers in a fully integrated form.