Oral squamous cell carcinoma (OSCC) is one of the major leading causes of cancer-related death worldwide, with limited effective markers for diagnosis and therapy, which has caused a low overall survival rate in the past decades. Kinases play important roles in tumor development and malignancy in various types of cancer. However, little is known about the role of kinases in OSCC cells. In this study, an arrayed kinome small interfering RNA (siRNA) library was used to screen oral cancer cell lines and counter assayed with normal fibroblast cells to identify the genes required for cancer cell proliferation. We found that polo-like kinase 1 (PLK1) was one of the most potent genes required for OSCC cell proliferation. The knockdown of PLK1 with a siRNA or antisense oligonucleotide (ASO) consistently diminished cyclin-B1 (CCNB1) expression/phosphorylation and the G₂-M phase transition. Similar effects were observed in cells treated with the PLK1 kinase inhibitor BI6727. Besides, The Cancer Genome Atlas (TCGA) analysis revealed that PLK1 was elevated in tumor tissues and associated with short survival in patients with OSCC. We also found that PLK1 expression was highly correlated with the expression of its downstream effector, CCNB1, in patients with OSCC. Coexpression of the two genes resulted in a poor prognosis of OSCC patients, particularly those in the advanced stages of OSCC. Taken together, our results suggest that PLK1 might be a diagnostic or therapeutic marker for OSCC.