The intracellular O-linked N-acetylglucosamine (O-GlcNAc) modification is known to be enriched in the nucleus and in particular on chromatin, but many of its chromatin targets remain to be identified. Herein we demonstrate the O-GlcNAcylation of YEATS Domain Containing 2 (YEATS2), a subunit of the chromatin Ada-two-A-containing (ATAC) complex and a reader of histone H3K27ac. We show that YEATS2 interacts with the O-GlcNAc transferase (OGT) and further pinpoint its major O-GlcNAcylation site to be Thr604 using electron transfer dissociation mass spectrometry. O-GlcNAcylation promotes the chromatin association of YEATS2, and the affinity between YEATS2 and other ATAC components on chromatin, such as ZZZ3, GCN5 and PCAF. Downstream, YEATS2-T604A attenuated the ATAC-dependent histone H3K9ac levels and inactivated the expression of essential ribosomal genes as shown in chromatin immunoprecipitation assays. Further, xenograft experiments show that YEATS2 O-GlcNAcylation promotes lung cancer tumorigenesis. Our work reveals the critical role of YEATS2 O-GlcNAcylation in stabilizing the ATAC complex on chromatin and expands the chromatin substrates of OGT.