<b><i>Background:</i></b> Mycosis fungoides (MF), the most common form of cutaneous T-cell lymphoma (CTCL), is a lymphoproliferative disorder characterized by proliferation of malignant T cells in a chronic inflammatory environment in the skin. The nature of MF is still not fully understood, but aberrant microRNA (miR) expression and function seem to play an important role in the pathogenesis and disease progression and have been proposed as a putative disease marker. Recent studies have reported aberrant expression of miR-93 in situ<i></i>in MF lesions and linked dysregulated miR-93 expression to advanced stages of MF. However, the pathophysiological role of miR-93 in MF is unknown. <b><i>Objective:</i></b> Here, we provide the first evidence that miR-93 targets the cell cycle regulator cyclin-dependent kinase inhibitor p21 and promotes growth of malignant T cells in MF. <b><i>Methods/Results:</i></b> Thus, inhibition of miR-93 in MF patient-derived malignant T-cell lines increases expression of p21 and inhibition of malignant proliferation. Notably, treatment with the histone deacetylase inhibitor Vorinostat (SAHA) reduces miR-93 expression and enhances p21 expression in the malignant T cells. Importantly, transfection with an miR-93 mimic partly blocks SAHA-induced p21 expression. <b><i>Conclusions:</i></b> we provide evidence that enhanced expression of the putative oncogenic miR, miR-93, represses the cell cycle inhibitor p21 and promotes proliferation of malignant T cells. Moreover, we demonstrate that SAHA triggers p21 expression – at least partly – through an inhibition of miR-93.