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Sickle cell disease (SCD) is caused by a mutation in the β-globin gene HBB. We used a custom adenine base editor (ABE8e-NRCH) to convert the SCD allele (HBB) into Makassar β-globin (HBB), a non-pathogenic variant. Ex vivo delivery of mRNA encoding the base editor with a targeting guide RNA into haematopoietic stem and progenitor cells (HSPCs) from patients with SCD resulted in 80% conversion of HBB to HBB. Sixteen weeks after transplantation of edited human HSPCs into immunodeficient mice, the frequency of HBB was 68% and hypoxia-induced sickling of bone marrow reticulocytes had decreased fivefold, indicating durable gene editing. To assess the physiological effects of HBB base editing, we delivered ABE8e-NRCH and guide RNA into HSPCs from a humanized SCD mouse and then transplanted these cells into irradiated mice. After sixteen weeks, Makassar β-globin represented 79% of β-globin protein in blood, and hypoxia-induced sickling was reduced threefold. Mice that received base-edited HSPCs showed near-normal haematological parameters and reduced splenic pathology compared to mice that received unedited cells. Secondary transplantation of edited bone marrow confirmed that the gene editing was durable in long-term haematopoietic stem cells and showed that HBB-to-HBB editing of 20% or more is sufficient for phenotypic rescue. Base editing of human HSPCs avoided the p53 activation and larger deletions that have been observed following Cas9 nuclease treatment. These findings point towards a one-time autologous treatment for SCD that eliminates pathogenic HBB, generates benign HBB, and minimizes the undesired consequences of double-strand DNA breaks.