The intermediate-conductance Ca²⁺-activated K⁺ channel K_Ca3.1 is involved in the promotion of tumor growth and metastasis, and is a potential therapeutic target for cancer. Higher K_Ca3.1 gene expression correlates with the shorter overall survival in cancer patients. Histone deacetylases (HDACs) post-translationally regulate the expression and activity of a number of proteins that play a crucial role in cancer development and progeression. We have showed that the K_Ca3.1 expression and activity are potentially reduced by the treatment with class I HDACs, HDAC2 and HDAC3 in prostate and breast cancer cell lines. Hypoxic tumor microenvironment is a common characteristic of solid cancers, and is associated with cancer metastasis and poor cancer prognosis. Hypoxia up-regulates the class III HDAC, SIRT1. Here we investigated the effect of the SIRT1 inhibitor on the K_Ca3.1 expression and activity in several types of cancer cells using real-time PCR, western blotting, flow cytometry, and voltage-sensitive fluorescence dye imaging assays. Pharmacological and siRNA-mediated inhibition of SIRT1 down-regulated K_Ca3.1 transcription and reduced its activity. These results suggest that SIRT1 may be a potential therapeutic target for K_Ca3.1-overexpressing cancers.