Clouds play a key role in Earth's radiative balance. Radiative transfer through liquid clouds depends on the optical properties of liquid water. Although these optical properties are known to be temperature dependent, radiative transfer simulations typically use properties based on measurements made at 298 K, even for supercooled liquid clouds at temperatures as low as 240 K. Here we report temperature‐dependent complex refractive indices (CRIs) of liquid water at 240, 253, 263, and 273 K from 1 to 15,000 cm−1 (0.7 to 10,000 μm). Imaginary parts of the CRI (k) are inferred from values reported in the literature for temperatures of 238 to 298 K and the real parts are obtained via Kramers‐Kronig transformation. Mie theory is used to calculate single scattering albedos and Legendre moments from the CRI, producing a set of optical constants suitable for radiative transfer calculations through supercooled liquid cloud at temperatures as low as 240 K. Ignoring the temperature dependence of complex refractive indices of liquid water is thereby found to result in biased‐high supercooled liquid‐cloud fluxes from 250 to 580 cm−1 and biased‐low fluxes from 710 to 940 cm−1. Plain Language Summary Clouds warm Earth's surface by trapping infrared light, that is, through the greenhouse effect. In cold, clean air, cloud droplets can stay liquid down to well below normal freezing temperature. Laboratory measurements have shown that the refractive indices of such “supercooled” liquid differ from those of warmer liquid. This has important consequences, since the refractive index influences a cloud's greenhouse effect. Here, we use a variety of laboratory measurements to infer the refractive indices of liquid water for temperatures of −33°C to 0°C (−28°F to 32°F). Key Points Temperature‐dependent complex refractive indices of liquid water are presented from 240 to 298 K and from 1 to 15,000 cm−1 Complex refractive indices of liquid water become more ice‐like as temperature decreases from 298 to 240 K The temperature dependence of liquid‐water refractive indices has important consequences for radiative transfer through supercooled clouds