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Mamyshev oscillators have pushed the frontiers in output parameters of ytterbium- and erbium-based ultrafast fiber oscillators in the spectral region around 1 µm and 1.5 µm within the last few years tremendously. In order to expand the superior performance toward the 2 µm spectral region, we present in this Letter an experimental investigation of the generation of high-energy pulses by a thulium-doped fiber Mamyshev oscillator. Generating highly energetic pulses is enabled by a tailored redshifted gain spectrum in a highly doped double-clad fiber. The oscillator emits pulses with an energy of up to 15 nJ, which can be compressed to 140 fs.