The invention relates to a device for amplifying a multi-wavelength pulse laser beam (2), which includes: a solid amplifier medium (1) having two planar surfaces, a front surface (12) and a reflective rear surface (11), a device for cooling the amplifier medium (1) via the rear surface (11) thereof. The front surface (12) of the amplifier medium (1) is inclined relative to the rear surface (11) of same by a first non-zero angle, and the device further includes a trapezoidal prism (4), having an input surface and an output surface which together form a second non-zero angle, the first and second angles being such that the beams of each wavelength are parallel to one another upon exiting the prism (4).