Glycogen synthase kinase beta (GSK3β) is considered as a promising target for lung cancer treatment and its inhibitor lithium chloride (LiCl) is widely regarded as having potent anti-proliferative and apoptosis-modulating activities. Proliferating cell nuclear antigen (PCNA), as an auxiliary protein for DNA polymerase delta, which regulates DNA replication and repair, has been reported to play an important role in regulating apoptosis. Here, we showed that GSK3β interacted with PCNA in H1299 lung adenocarcinoma cells using GST pull-down and co-immunoprecipitation experiments. We discovered that their interaction can be enhanced within the first 3 h after UVC irradiation and decreased gradually with time. Overexpression of PCNA protein decreased GSK3β Ser9 phosphorylation, whereas knockdown of PCNA using small interfering RNA (siRNA) increased Ser9 phosphorylated GSK3β, which was attenuated by phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002 after UVC irradiation, indicating the involvement of the PI3K-AKT pathway. Functional analyses suggested that downregulation of PCNA sensitized H1299 cells to LiCl-induced apoptosis. Thus, our results unraveled a novel regulatory of GSK3β by PCNA and provided a promising direction for treatment of lung cancer.