We report the first (to our knowledge) wafer-fused high-power passively mode-locked semiconductor disk laser operating at $1.57\mu \mathrm{m}$ wavelength. An InP-based active medium was fused with $\mathrm{Ga}\mathrm{As}∕\mathrm{Al}\mathrm{Ga}\mathrm{As}$ distributed Bragg reflector on a $2\text{\hspace{0.17em}inch}$ wafer level, resulting in an integrated monolithic gain mirror. An intracavity wedged diamond heat-spreader capillary bonded to the gain chip provides efficient heat removal from the gain structure without disturbing the spectrum of the mode-locked laser. The laser produces over $0.6\mathrm{W}$ of average output power at $15°\mathrm{C}$ with $16\mathrm{ps}$ pulse width. The total output power accounting for all output beams emerging from the cavity was $0.86\mathrm{W}$ . The results reveal an essential advantage of wafer fusion processing of disparate materials over monolithically grown InP-based gain structures and demonstrate the high potential of this technique for power scaling of long-wavelength semiconductor disk lasers.