Inhibition of histone deacetylase (HDAC) activity by HDAC inhibitors (HDACis) results in cancer cell growth inhibition, and HDACis have been revealed as potential anti-skin cancer agents. p21 is a cyclin-dependent kinase inhibitor and an essential regulator of growth inhibition. Recently, we reported that activating transcription factor 3 (ATF3) could significantly promote skin cancer cell growth. This study explored the relationship between ATF3 and HDACi-induced growth inhibition of epidermoid carcinoma cells. We found that trichostatin A (TSA) treatment inhibited cell growth in A431 epidermoid carcinoma cells in a dose-dependent manner. Simultaneously, p21 and ATF3 expression levels were upregulated and downregulated upon TSA stimulation, respectively. ATF3 overexpression promoted cell growth and downregulated p21 expression. In contrast, ATF3 depletion resulted in cell growth reduction and p21 transcriptional upregulation. More importantly, ATF3 overexpression partially antagonized TSA-induced growth inhibition and p21 activation. Collectively, these data demonstrate that ATF3 acts as an essential negative regulator of TSA-induced cell growth inhibition through interfering with TSA-induced p21 activation.