The diamondback moth, Plutella xylostella (L.), is a major pest that has developed resistance to many groups of synthetic insecticides. Natural plant products present reliable alternatives for its management. In this study, the activities of essential oils and ethanol extracts from three medicinal plants, i.e., Curcuma longa L. (Zingiberaceae), Cymbopogon nardus (L.) Rendle (Gramineae), and Acorus calamus L. (Acoraceae), from Thailand against P. xylostella were evaluated. Four concentrations of the extracts (i.e., 0.625, 1.25, 2.5, and 5%) were tested. The essential oils and ethanol extracts of Cu. longa, Cy. nardus, and A. calamus exhibited different degrees of activity against P. xylostella. Of these, both the essential oil and ethanol extract of A. calamus at 5% (v/v) concentration exhibited feeding toxicity for P. xylostella larvae, resulting in 100% mortality 1 d after treatment, with a median lethal concentration of 0.528% for the essential oil and 1.074% for the ethanol extract. Furthermore, all concentrations of A. calamus ethanol extract exhibited contact toxicity to the larvae, resulting in >75% mortality 3 d after treatment. All tested extracts, except A. calamus ethanol extract at the 0.625% concentration, exhibited high repellent activity against P. xylostella larvae, while Cu. longa and Cy. nardus ethanol extracts at 5% (w/v) concentration and A. calamus essential oil at 2.5% (v/v) concentration deterred oviposition by P. xylostella adults with an effective repellency >70%. These results indicate that both essential oils and ethanol extracts of these plants have the potential to be developed as insecticides for use in integrated pest management of P. xylostella.