Prolactinomas are the most prevalent functional pituitary adenomas that cause chronic pathological hyperprolactinemia. Prolactin is known to promote cell growth and inhibit apoptosis in cells. Paeoniflorin is the principal component of radix Paeoniae alba (the main ingredient in some traditional herbal formulas clinically used for hyperprolactinemia-associated disorders). Recent findings from intensive studies have suggested that paeoniflorin regulates cell proliferation and apoptosis in many cell lines. However, the effects of paeoniflorin in pituitary tumor cells remain unknown. Here the results by the Cell Counting Kit-8 and colony formation assays showed that paeoniflorin concentration-dependently decreased cell viability in both MMQ and GH3 cells and colony formation in GH3 cells, suggesting inhibition of cell proliferation by paeoniflorin. By flow cytometry, paeoniflorin was found to increase apoptotic rate in MMQ cells. Mechanistically, Western blot results revealed that paeoniflorin enhanced protein expression of cleave caspase-9 and -3, and Bax, whereas it suppressed Bcl-2 protein expression in MMQ cells. Furthermore, paeoniflorin upregulated phosphorylated p53 protein expression, but it decreased prolactin concentration and prolactin protein expression in both MMQ and GH3 cells. Thus, the present results demonstrate that paeoniflorin inhibits cell proliferation and induces the mitochondrial pathway-mediated apoptosis in prolactinoma cells. These antitumor property is associated with inhibition of prolactin secretion. Our findings may provide new insight into the mechanisms underlying improving prolactinoma-associated disorders of paeoniflorin-enriched herbs and formulas.