Dictyopteris divaricata (DD) has been reported to exert diverse pharmacological activities, including anti-inflammatory, antioxidant, and anticancer effects. In this study, we aimed to investigate the anticancer potential of the ethanolic extract of DD (EDD) in non-small cell lung cancer (NSCLC) cells and to explore the underlying mechanism. EDD significantly suppressed cell proliferation in H1299, PC9, and H1975 NSCLC cells. EDD treatment increased the proportion of Annexin V-positive cells and cells in sub-G1 phase, indicating the induction of apoptosis. This observation was further supported by the presence of fragmented nuclei and increased expression of cleaved PARP and cleaved caspase-3 in NSCLC cells following EDD treatment. Mechanistically, EDD decreased the phosphorylation levels of signal transducer and activator of transcription 3 (STAT3) and Src. Transfection of constitutively activated STAT3 into H1975 cells partially attenuated EDD-induced apoptosis, highlighting the contribution of STAT3 inhibition to the anticancer activity of EDD. In addition, we identified fucosterol as a major constituent of EDD that exhibited similar anticancer potential in NSCLC cells. Taken together, our results demonstrate that EDD induces apoptosis in NSCLC cells by inhibiting STAT3 activity. We propose EDD as a potential candidate for the development of therapies targeting NSCLC.