Optical chirality is a fundamental property of light that plays a crucial role in chiroptical spectroscopy, analytical chemistry, spin photonics, and astrobiology. However, giant optical chirality at the wavelength beyond 15 μm remains untapped, which hinders its various applications, such as ultrasensitive vibrational circular dichroism, in the mid-infrared region. Here we propose and theoretically present giant optical chirality with a, to our knowledge, record-high value of ∼330 at the wavelength beyond 15 μm in a diamond metasurface. This is enabled by tailoring the multipole degeneracy and leveraging the low optical loss of the diamond metasurface. This work provides a new method for boosting the optical chirality at the wavelength beyond 15 μm.