Abstract: Histone deacetylase 9 (HDAC9) belongs to the IIa subtype of HDACs. It is responsible for changing the structure of chromosomes and regulating the transcription of genes by catalyzing the deacetylation of H3, H4, and non-histone proteins in vivo. Emerging studies have demonstrated that HDAC9 is closely related to tumors, but their expression and function in different tumors is not the same. This could ultimately lead to the opposite effect of promoting or suppressing tumorigenesis; unfortunately, the mechanisms are not clear. Recently, epigenetic treatment with HDAC deacetylases inhibitors (HDACIs) has become a hot topic and the development of selective HDACIs in combination with chemotherapy, radiotherapy, and immunotherapy has gained traction. However, studies targeting HDAC9 are limited. This review summarizes the recent studies about HDAC9 in tumors.