Polaritons are hybrid light-matter quasi-particles that have gathered a significant attention for their capability to show room temperature and out-of-equilibrium Bose-Einstein condensation. More recently, a novel class of ultrafast optical devices have been realized by using flows of polariton fluids, such as switches, interferometers and logical gates. However, polariton lifetimes and propagation distance are strongly limited by photon losses and accessible in-plane momenta in usual microcavity samples. In this work, we show experimental evidence of the formation of room temperature propagating polariton states arising from the strong coupling between organic excitons and a Bloch surface wave. This result, which was only recently predicted, paves the way for the realization of polariton devices that could allow lossless propagation up to macroscopic distances.Mixed light-matter excitations, also called polaritons, arise from the non-perturbative coupling of optical transitions with large oscillator strength and electromagnetic modes propagating or confined in semiconducting or insulating media.[1, 2] Recently, polariton excitations in two-dimensional geometries have been attracting considerable interest, both for fundamental studies on out-of-equilibrium Bose-Einstein condensation and superfluidity, [3][4][5] and for the possibility to exploit their nonlinear properties for all-optical operations. [6,7] Elementary excitations can be observed in materials such as squaraine, porphyrin and cyanine dyes. As opposed to Wannier-type exciton polaritons, their ultra-high oscillator strength allows for the observation of polariton effects at room temperature and for Rabi splittings that could reach energies of the order of the excitonic transistion.So far, most of the research on polaritons, concerning both Wannier and organic types, [8,9] has been mainly focused on the strong coupling to the photonic modes of high-finesse planar microcavities, where the active medium is embedded in a cavity layer between two highreflectivity mirrors. However, planar microcavities offer little freedom in engineering the propagation of polaritons, due to their small accessible momenta. Recently, it has been suggested that similar phenomena could also be observed at the interface between a single Bragg mirror (DBR) and a homogeneous medium by exploiting strong light-matter coupling to Bloch surface waves. [10,11] Such Bloch surface wave polaritons (BSWP), [12] i.e. mixed excitations bound to the surface of the Bragg mirror,-which exploit the confinement of the optical mode due to total internal reflection as weel as a photonic bandgap-could be exploited for prospective polariton devices with high efficiency and controlled long range propagation, or for applications requiring high surface sensitivity such as optical sensors, [13,14] with clear advantages over alternative structures where metal deposition is required. [15,16] In this letter we report on the experimental demonstra- tion of strong light-matter coupling between J-aggregate ex...