Context. Current star formation models imply that the binary fraction of Population III stars is non-zero. The evolution of these binaries must have led to the formation of compact object binaries. Aims. We estimate the gravitational wave background originating in these binaries and discuss its observability. Methods. The properties of the Population III binaries are investigated using a binary population synthesis code. We numerically model the background and take into account the evolution of eccentric binaries. Results. The gravitational wave background from Population III binaries dominates the spectrum below 100 Hz. If the binary fraction is larger than 10-2, the background will be detectable by Einstein Telescope (ET), Laser Interferometer Space Antenna (LISA), and DECi-Hertz Interferometer Gravitational wave Observatory (DECIGO). Conclusions. The gravitational wave background from Population III binaries will dominate the spectrum below 100 Hz. The instruments LISA, ET, and DECIGO should either see it easily or, in the case of non-detection, provide very strong constraints on the properties of the Population III stars.