Manipulation of light using subwavelength waveguides is a key technology in the development of miniaturized photonic circuits, which possess various advantages over their electronic counterparts. The novel approach presented for such waveguiding involves the propagation of exciton polaritons (EPs), which are quasi-particles formed by strong exciton–photon coupling, along organic dye nanofibers. A self-assembled nanofiber of thiacyanine (TC) with a width of ≈200 nm propagates the EPs created by an optical excitation over a submillimeter-scale distance and passes through a bend with a micrometer-scale radius with low bending loss. To demonstrate the remarkable potential of EP-based miniaturized photonic circuits, asymmetric Mach–Zehnder interferometers (AMZIs) are fabricated with TC nanofibers by micromanipulation. The AMZIs with a footprint of ≈20 μm × 20 μm exhibit a visibility of nearly unity and function as channel drop filters with the considerably high extinction ratio of up to ≈15 dB. Such high-performance and ultracompact channel drop filters operating in the visible wavelength region have rarely been developed with other waveguide technologies. The coherent properties of the EPs in the nanofibers are investigated using time-resolved experiments. The coherent properties provide useful information for designing EP-based photonic circuits and for understanding EP dynamics in a nanofiber.