Purified recombinant fungal immunomodulatory protein from Ganoderma tsugae (reFIP-gts) has anti-telomerase effects in human lung adenocarcinoma A549 cells. However, how reFIP-gts affects cancer cell fates remains unclear. Here, we demonstrated that reFIP-gts-treated lung cancer cells are arrested at G1 phase by flow cytometry and possess morphological phenotype consistent with cellular senescence. The senescent nature of these cells was supported by positive staining for senescence-associated beta-galactosidase activity and increased lysosomal content in A549 and CaLu-1 lung cancer cells. Arrest of cells at G1 appears to be the key means through which reFIP-gts induces premature cellular senescence in A549 cells. Finally, reFIP-gts- treated A549 cells grew more slowly and formed significantly fewer cell colonies in soft agar than untreated A549 cells. In an in vivo mouse model, A549 cells treated with reFIP-gts grew significantly slower than cells treated with PBS alone, confirming that lung tumor can be inhibited by reFIP-gts. The use of reFIP-gts may be a powerful new strategy for chemoprevention and antineoplastic therapy.