Nitidumpeptins A and B (1 and 2), two novel cyclic hexapeptides, were isolated from the herb Zanthoxylum nitidum var. tomentosum. Their planar structures were elucidated based on NMR and MS spectrometric analysis, and the absolute configurations were determined by the Marfey's method. Structurally, 1 is a unique peptide with a backbone bearing a pyrrolidine-2,5-dione unit, which is the first occurrence moiety specifically in a naturally occurring cyclohexapeptide. The total synthesis of 1 and 2 was achieved by solution-phase in parallel with solid-phase peptide synthesis, and their absolute configurations were further confirmed. The combination of 2 with gefitinib exhibited synergistic antiproliferative activity in acquired gefitinib-resistant non-small cell lung cancer cells (HCC827-gef). The underlying mechanism for the antiproliferative activity of 2 was in part associated with the suppression of YAP expression in HCC827-gef cells.