Histone lysine β-hydroxybutyrylation (Kbhb) is an epigenetic mark linking ketone metabolism to transcription. However, the molecular mechanism by which β-hydroxybutyrate is converted to β-hydroxybutyryl–coenzyme A (BHB-CoA), the cofactor for Kbhb, remains unknown. Here, we report that acetyl-CoA synthetase short-chain family member 2 (ACSS2) coupled with lysine acetyltransferase 7 (KAT7) modulates β-hydroxybutyrylation on lysine 9 of histone H3 (H3K9bhb) to promote transcription. We show that KAT7 serves as a β-hydroxybutyryltransferase and preferably catalyzes histone Kbhb, especially H3K9bhb, in 1171 identified Kbhb substrates. ACSS2 is a BHB-CoA synthetase. This enzyme can sense cellular β-hydroxybutyrate and translocate into the nucleus, where it binds to and colocalizes with KAT7 at specific locus of chromatin. The ACSS2-generated BHB-CoA can fuel KAT7 for histone H3K9bhb. We demonstrate that the β-hydroxybutyrate drives the ACSS2-KAT7-H3K9bhb axis to promote epigenetic regulation and tumor cell growth. Our study not only identifies the founding member of BHB-CoA ligase but also reveals the mechanism underlying KAT7-catalyzed histone Kbhb using ACSS2-generated BHB-CoA.