The space-resolved Gouy phase shift for focused surface plasmon polaritons (SPPs) in the optical regime is experimentally demonstrated for the first time. SPPs are excited by CW laser radiation focused onto a dielectric ridge placed on a cover glass surface coated by a silver film. The Gouy phase measurements are performed using interference between a reference plane wave and leakage radiation. The measured Gouy shift has a value of π/2 corresponding to the two-dimensional nature of surface plasmon polaritons.