We present the experimental demonstration of a superconducting photon number resolving detector. It is based on the series connection of N superconducting nanowires, each connected in parallel to an integrated resistor. The device provides a single voltage readout, proportional to the number of photons detected in distinct nanowires. Clearly separated output levels corresponding to the detection of n = 1-4 photons are observed in a 4-element detector fabricated from an NbN film on GaAs substrate, with a single-photon system quantum efficiency of 2.6 at λ = 1.3 μm. The series-nanowire structure is promising in view of its scalability to large photon numbers and high efficiencies.