A fiber‐based approach that generates mid‐infrared femtosecond pulses in the 3–4 m spectral region with microjoule‐level single pulse energy is demonstrated. This is realized in a piece of gas‐filled antiresonant hollow‐core fiber that is pumped by a two‐micron light source. A rapid variation of the dispersion near a structural resonance of the fiber creates a phase‐matching point in mid‐infrared, which mediates the frequency‐down conversion. Femtosecond pulses centered at 3.16 m wavelength with the pulse energy of more than 1 J are generated, achieving a conversion efficiency as high as 8.2%. The emission wavelength is determined solely by the dielectric wall thickness of cladding elements, while the yield is subject to other experimental parameters. This, combined with high power‐handling capability of hollow‐core fibers, makes it possible to power scale the mid‐infrared output by either increasing the pulse energy or repetition rate of the pump. The technique presents a new pathway to build an all‐fiber‐based mid‐infrared supercontinuum source, which promises to be a powerful new tool for ultrahigh sensitivity molecular spectroscopy.