We report experimental observations of a large Bragg reflection from arrays of cold atoms trapped near a one-dimensional nanoscale waveguide. By using an optical lattice in the evanescent field surrounding a nanofiber with a period nearly commensurate with the resonant wavelength, we observe a reflectance of up to 75% for the guided mode. Each atom behaves as a partially reflecting mirror and an ordered chain of about 2000 atoms is sufficient to realize an efficient Bragg mirror. Measurements of the reflection spectra as a function of the lattice period and the probe polarization are reported. The latter shows the effect of the chiral character of nanoscale waveguides on this reflection. The ability to control photon transport in 1D waveguides coupled to spin systems would enable novel quantum network capabilities and the study of many-body effects emerging from long-range interactions. DOI: 10.1103/PhysRevLett.117.133603 In recent years, the coupling of one-dimensional bosonic waveguides and atoms, either real or artificial, has raised a large interest [1][2][3]. Beyond the remarkable ability to couple a single emitter to a guided mode [3], the 1D reservoir would also enable the exploration and eventual engineering of photon-mediated long-range interactions between multiple qubits, a challenging prospect in free-space geometries. This emerging field of waveguide quantum electrodynamics promises unique applications to quantum networks, quantum nonlinear optics, and quantum simulation [4][5][6].In this context, progress has been reported on various fronts. In the microwave regime, the coupling of superconducting qubits to a one-dimensional transmission line provides a versatile platform to study such photon-mediated interactions [2]. At optical frequencies, recent experimental advances include the development of 1D nanoscale dielectric waveguides coupled to cold atoms trapped in the vicinity [7][8][9]. In these experiments, tight transverse confinement of the electric field achieves an effective mode area comparable to the atomic cross section and thereby a strong atom-photon interaction in a single-pass configuration [10].Coupling of atom arrays to 1D waveguides could lead to a variety of remarkable cooperative phenomena. This coupling can strongly modify the photon transport properties [11][12][13], resulting for instance in sub-and superradiant decays as recently observed for two coupled atoms [14]. It can also lead to photonic band gaps and provide atomic Bragg mirrors, with envisioned applications to integrated cavity QED [15][16][17]. This setting is as well at the basis of a recently proposed deterministic state engineering protocol [18] and constitutes the building block of chiral spin networks in which the emission into the left-and rightpropagating modes is asymmetric [19]. Moreover, strong optomechanical couplings resulting from photon-mediated forces can give rise to rich spatial atomic configurations, including self-organization [20,21].Optical nanofibers offer a promising platform for exploring t...