Abstract The surface modification of superconducting radio frequency (SRF) cavities is mandatory to further push the limits in future accelerators. One strategy is the deposition of multilayer superconducting and insulating materials on top of the inner surface of an SRF cavity. Here, we report on a successful low-temperature coating of an SRF cavity with insulating A l 2 O 3 by thermal atomic layer deposition (ALD), without mitigating its maximum achievable accelerating field of more than 40 MV m⁻¹. Furthermore, an improvement of the surface resistance above 30 MV m⁻¹ has been observed, which is likely caused by an enhanced oxygen diffusion during the deposition process. Our results show that ALD is perfectly suited to provide a conformal coat to the interior of the cavity and to even modify and improve the properties of such devices.