FK228 (Romidepsin) and suberoylanilide hydroxamic acid (SAHA, Vorinostat) are FDAapproved histone deacetylase inhibitors (HDACi) for Cutaneous T-cell Lymphoma (CTCL), including the leukemic subtype Sézary Syndrome (SS). This study investigates RAD23B and STAT3 gene perturbations in a large cohort of primary Sézary cells and the effect of FK228 treatment on tyrosine phosphorylation of STAT3 (pYSTAT3) and RAD23B expression. We report RAD23B copy number variation in 10% (12/119; p ≤ 0.01) of SS patients, associated with reduced mRNA expression (p = 0.04). RAD23B knockdown in a CTCL cell line led to a reduction in FK228-induced apoptosis. HDACi treatment significantly reduced pYSTAT3 in primary Sézary cells and was partially mediated by RAD23B. A distinct pattern of RAD23B-pYSTAT3 co-expression in primary Sézary cells was detected. Critically, Sézary cells harbouring the common STAT3 Y640F variant were less sensitive to FK228-induced apoptosis and exogenous expression of STAT3 Y640F and D661Y conferred partial resistance to STAT3 transcriptional inhibition by FK228 (p ≤ 0.0024). These findings suggest that RAD23B and STAT3 gene perturbations could reduce sensitivity to HDACi in SS patients.