We report, to the best of our knowledge, the first mode-locked fiber laser to operate in the femtosecond regime well beyond 3 µm. The laser uses dual-wavelength pumping and nonlinear polarization rotation to produce 3.5 µm wavelength pulses with minimum duration of 580 fs at a repetition rate of 68 MHz. The pulse energy is 3.2 nJ, corresponding to a peak power of 5.5 kW.