We report the first demonstration, to our knowledge, of the femtosecond laser operation by using a new alloyed Yb:GYSO crystal as the gain medium. With a $5\mathrm{at.}\%$ ${\mathrm{Yb}}^{3+}$ -doped sample and chirped mirrors for dispersion compensation, we obtained pulses as short as $210\mathrm{fs}$ at the center wavelength of $1093\mathrm{nm}$ . The average mode-locking power is $300\mathrm{mW}$ , and the pulse repetition frequency is $80\mathrm{MHz}$ .