A core-pumped ytterbium-doped fiber laser is presented, tunable over 26 nm from 1017 nm to 1043 nm. A diffraction grating pair in Littman-Littrow configuration in the unidirectional ring-cavity provides the wavelength adjustment. The laser resonator with a free spectral range of 260 MHz supported stable single frequency operation with a maximum linear polarized output power of 31 mW.