Passive mode locking of a diode-pumped ${\mathrm{Yb}}^{3+}:{\mathrm{KYF}}_4$ laser is demonstrated using a semiconductor saturable absorber mirror. A high-stability, transform-limited pulse train with a repetition rate of $57\mathrm{MHz}$ is generated. Solitonlike pulses with maximum average output power of $250\mathrm{mW}$ , minimum pulse duration of $170\mathrm{fs}$ , and rms time jitter of $360\mathrm{fs}$ were obtained.