A novel alkali extractable polysaccharide (designated as PNA-2) was purified from Pleurotus nebrodensis and the effects of purified PNA-2 on the proliferation and apoptosis of human hepatic cancer cells (HepG2) were investigated in this study. The results of a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay indicated that PNA-2 inhibited the proliferation of HepG2 cells by apoptosis induction, which was also characterized using scanning electron microscopy (SEM). Moreover, the expression of apoptosis-associated mRNA, proteins and the cell-cycle arrest at the G0/G1 phase was determined using RT-qPCR, Western blot and flow cytometry, respectively. A notable inhibition of the migration rate of PNA-2-treated HepG2 cells was observed using a cell scratch assay. DNA damage was observed using a comet assay and AO/EB staining in HepG2 cells, which were exposed to PNA-2. Induction of the mitochondria-mediated intrinsic apoptotic pathway by PNA-2 was indicated by the loss of mitochondrial membrane potential (ΔΨm), Bcl-2 dysregulation and cytochrome c release. All the results suggested that the mitochondria-mediated intrinsic apoptotic pathway could be involved in PNA-2-mediated apoptosis of human liver carcinoma cells HepG2. Finally, the results indicated that PNA-2 significantly suppressed tumor growth in HepG2 tumor-bearing mice, indicating that PNA-2 may be developed as a candidate drug or functional food factor to prevent or treat liver cancer.