Surface plasmon polaritons (SPPs) may serve as ultimate data processing expedients in future nanophotonic applications. SPPs combine the high localization of electrons with the bandwidth, frequency and propagation properties of photons, thus supplying nature with the best of two worlds. However, although plasmonics have recently gained constantly growing scientific attention, logic devices that operate on SPPs on a deep nanometer scale are yet to be demonstrated. Here, we design, fabricate and experimentally verify the smallest, first ever reported all optical nanoplasmonic XOR logic gate. The introduced XOR device is based on a novel engineerable interferometry scheme with extremely compact dimensions of λ3/15500, which can be used to realize a variety of plasmonic logic functionalities. We use frequency modulated Kelvin probe microscopy to provide evidence of binary XOR functionality performed directly on SPPs with λ3/80000 mode volumes. An extinction ratio of 10 dB is achieved for a device length of 150 nm, increasing up to 30 dB for a device length of 280 nm. Our findings confirm plasmonics as the favorite data carriers in integrated all optical logic devices operating on the deep nanoscale, and pave the way to the development of future ultrafast information processing technologies based on SPPs.