Background: Polygonum cuspidatum (Hu Zhang) is a polyphenol-rich botanical in which resveratrol occurs at low levels mainly as polydatin. In this study, we generated a microbially converted P. cuspidatum extract (CPE) with markedly enriched resveratrol content from the unconverted P. cuspidatum extract (PE), and evaluated its anticancer efficacy and safety in comparison with those of the unconverted extract. Methods: High-performance liquid chromatography (HPLC), A549 cell-based assays, an A549 xenograft model, and a 28-day repeated-dose rat study were performed. Results: HPLC showed near-complete depletion of polydatin, marked enrichment of resveratrol, and a modest reduction in emodin. CPE more effectively inhibited A549 cell proliferation and migration than PE, suppressed NLRP3 inflammasome and nuclear factor kappa B (NFκB) signaling, and promoted reactive oxygen species (ROS) accumulation and mitochondrial apoptosis. Oral CPE also significantly reduced tumor growth in an A549 xenograft model. In a 28-day repeated-dose rat study, no treatment-related abnormalities were observed in blood biochemistry or histopathology. Conclusions: These findings support CPE as a promising functional food ingredient.