Introduction. Psoralen is a main active component of Psoralea corylifolia Linn. (Leguminosae). Psoralen has been reported to show antitumor effects and activity to accelerate osteoblastic proliferation. Nevertheless, the antitumor mechanism of psoralen in osteosarcoma has never been elucidated. The current study is aimed to investigate the therapeutic function of psoralen in human osteosarcoma cells and its potential regulatory mechanism. Material and methods. Effects of psoralen (0–70 μg/mL) on the viability of two osteosarcoma cell lines cultured for 48 h was evaluated by MTT assays. The concentration of IC10 (8 μg/mL for MG-63 cells and 9 μg/mL for U2OS cells) was regarded to be a non-cytotoxic dose selected as the working concentration in the subsequent experiments. Effects of psoralen on cell proliferation for 48 h was assessed by colony formation assays. Flow cytometry analyses were performed to measure cell cycle and apoptosis. RT-qPCR and Western blotting were carried out to assess RNA expression and protein levels of endoplasmic reticulum (ER) stress associated factors. Results. Psoralen inhibited osteosarcoma cell viability (IC50 25 μg/mL for MG-63 cells and IC50 40 μg/mL for U2OS cells) in a dose-dependent manner and growth inhibition rate reached the highest level when cells were treated with 70 μg/mL psoralen. Psoralen induced cell cycle arrest in the G0/G1 phase and promoted apoptosis of both MG-63 and U2OS cells. The treatment of psoralen resulted in an increase in ATF-6 and CHOP protein levels as well as a decrease in Bcl-2 protein level, indicating that cell apoptosis induced by psoralen was associated with ER stress. Treatment with 4-PBA, the ER stress inhibitor, attenuated the ability of psoralen to promote apoptosis of MG-63 and U2OS cells. Conclusions. Psoralen showed growth-inhibitory effects in osteosarcoma cells, and induced apoptosis via the ER stress pathway, which might be a potential drug to suppress the development of osteosarcoma.