In the present study, the effect of an extract of immature Prunus salicina Lindl. cv. Soldam fruit on the viability and induction of apoptosis in human hepatocellular carcinoma HepG2 cells was investigated. The results showed that in comparison with other cancer cells, the growth inhibition exerted by immature plum extracts was greatest in HepG2. Apoptosis in HepG2 cells mediated by immature plums was associated with "death receptor signaling." Immature plum extracts significantly increased the activation of caspase-8, -10, and -3 and expression of the caspase-3 target proteins alpha-fodrin (induces membrane blebbing and cell shrinkage), poly(ADP-ribose) polymerase (a nuclear enzyme that is involved in DNA repair following DNA nicks), and DNA fragmentation factor (induces apoptotic DNA fragmentation). The total yield of identified polyphenols in immature plum extract was 10 g/kg dry weight. The major components, (-)-epicatechin and (-)-gallocatechin gallate, were 34.7% and 28.6% of total polyphenols, respectively. (+)-Catechin, (-)-epicatechin gallate, and (-)-catechin gallate were also found. On the basis of these results, the immature plum (P. salicina Lindl. cv. Soldam) and its active compound, (-)-epicatechin, are expected to be a natural resource for developing novel therapeutic agents for cancer prevention and treatment.