Surface plasmon polaritons (plasmons) are a combination of light and a collective oscillation of the free electron plasma at metal/dielectric interfaces(1). This interaction allows subwavelength confinement of light beyond the diffraction limit inherent to dielectric structures(2). As a result, the intensity of the electromagnetic field is enhanced, with the possibility to increase the strength of the optical interactions between waveguides, light sources(3-6) and detectors(7,8). Plasmons maintain non-classical photon statistics(9,10) and preserve entanglement upon transmission through thin, patterned metallic films(11,12) or weakly confining waveguides(13). For quantum applications(3,14), it is essential that plasmons behave as indistinguishable quantum particles. Here we report on a quantum interference experiment in a nanoscale plasmonic circuit consisting of an on-chip plasmon beamsplitter with integrated superconducting single-photon detectors(15) to allow efficient single plasmon detection(16). We demonstrate a quantum-mechanical interaction between pairs of indistinguishable surface plasmons by observing Hong-Ou-Mandel (HOM) interference(17), a hallmark non-classical interference effect that is the basis of linear optics-based quantum computation(18). Our work shows that it is feasible to shrink quantum optical experiments to the nanoscale and offers a promising route towards subwavelength quantum optical networks.