Abstract The complex refractive index (CRI) of ice is one of the main parameters driving the optical properties of ice clouds and frozen surfaces, hence their climate impact. While already characterized at short wavelengths, ice CRI has never been measured across the infrared range at temperatures relevant to Earth. Here, we present the first comprehensive experimental determination of the temperature‐dependent CRI of hexagonal ice in the infrared (25–5,000 cm−1 $mathrm{c}{mathrm{m} }^{-1}$), and for temperatures 150–270 K. The method relies on high‐accuracy synchrotron‐based transmission measurements, combined with a modeling of the optical system and an advanced retrieval procedure. The temperature dependence of the bands of ice, particularly the connectivity band, have been measured for the first time, highlighting differences from existing extrapolated databases often exceeding 20% $%$ in strongly temperature depending regions. The obtained data set fills major gaps in both the spectral and temperature dimensions, and will help improve models of frozen surfaces and ice clouds.