Surface-plasmon modes confined at the interface between a metal and a semiconductor are exploited in place of conventional dielectric waveguides for the realization of a λ∼17 μm semiconductor laser. The device is based on the quantum cascade concept and outperforms with its 38 mW of peak output power and 240 K of maximum operating temperature any previous semiconductor laser of comparable wavelength. Pure single-wavelength emission with a tuning rate of ∼1 nm/K is achieved using Bragg reflection from a two-metal grating that modulates the skin depth of the surface plasmons.