Lambertianic acid (LA) is a biologically active compound from the leaves of Pinus koraiensis. In the present study, apoptotic mechanisms of LA plus TNF-related apoptosis-inducing ligand (TRAIL) were elucidated in non-small cell lung cancer cells (NSCLCs). Cytotoxicity assay, flow cytometry, immunoprecipitation, and Western blotting were performed. Here, combined treatment of LA and TRAIL increased cytotoxicity, sub-G1 population, cleaved poly (ADP-ribose) polymerase (PARP), and caspase3/8/9 in A549 and H1299 cells compared to LA or TRAIL alone. Furthermore, combined treatment of LA and TRAIL significantly decreased antiapoptotic proteins such as B-cell lymphoma 2 (Bcl-2), Fas-like inhibitor protein (FLIP), and X-linked inhibitor of apoptosis protein (XIAP), and enhanced the activation of proapoptotic proteins Bid compared to LA or TRAIL alone. In addition, combined treatment of LA and TRAIL upregulated the expression of Death receptor 4 (DR4) and downregulated phosphorylation of nuclear factor κ-light-chain-enhancer of activated B cells (p-NF-κB), inhibitory protein of kB family (p-IκB), and FLIP in A549 and H1299 cells along with disrupted binding of XIAP with caspase3 or NF-κB. Overall, these findings suggest that lambertianic acid enhances TRAIL-induced apoptosis via inhibition of XIAP/NF-κB in TRAIL resistant NSCLCs.