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Lithium niobate (LiNbO3, LN) is a ferroelectric crystal of interest for integrated photonics owing to its large second-order optical nonlinearity and the ability to impart periodic poling via an external electric field. However, on-chip device performance based on thin-film lithium niobate (TFLN) is presently limited by propagation losses arising from surface roughness and corrugations. Atomic layer etching (ALE) could potentially smooth these features and thereby increase photonic performance, but no ALE process has been reported for LN. Here, we report an isotropic ALE process for x-cut MgO-doped LN using sequential exposures of H2 and SF6/Ar plasmas. We observe an etch rate of 1.59±0.02 nm/cycle with a synergy of 96.9%. We also demonstrate that ALE can be achieved with SF6/O2 or Cl2/BCl3 plasma exposures in place of the SF6/Ar plasma step with synergies of 99.5% and 91.5%, respectively. The process is found to decrease the sidewall surface roughness of TFLN waveguides etched by physical Ar+ milling by 30% without additional wet processing. Our ALE process could be used to smooth sidewall surfaces of TFLN waveguides as a postprocessing treatment, thereby increasing the performance of TFLN nanophotonic devices and enabling new integrated photonic device capabilities.