Updated project metadata. B lymphopoiesis is a key developmental event orchestrated by a complex combinatorial action of lineage-specific transcription factors. In early B cell progenitors, lineage commitment is directly mediated by the master regulator PAX5, whose deficiency is commonly associated with B cell Acute Lymphoblastic Leukemia (B-ALL). Despite its essential role in mammalian immunity, the regulatory mechanisms that control PAX5 function remain largely unknown. Here we show that NAD+-dependent enzyme SIRT7 coordinates B cell development progression through PAX5. We have identified a SIRT7-dependent regulatory switch based on dynamic deacetylation of a single PAX5 residue, which controls its activity and thereby B cell fate. While a PAX5K198 acetylated mimic is incapable of inducing both B cell development and identity due to reduced protein stability and impaired binding to chromatin, deacetylation of this residue boosts PAX5 activity, leading to massive gene repression and in vivo restoration of B cell commitment but not differentiation. These findings suggest an unexpected uncoupling of hematopoietic differentiation and lineage commitment. Further supporting the functional relevance of the SIRT7-PAX5 axis, the interplay between both factors is conserved in human B-ALL, where high SIRT7 expression is an independent good prognostic factor. Our findings unveil a crucial mechanism in the regulation of B cell production based on the control of PAX5 function and underscore the key role of Sirtuins in the regulation of the immune system.