The composition of the essential oil isolated from the rhizomes of Zingiber zerumbet Smith was determined using GC and GCâMS. Zerumbone (74.82%), humulene (6.02%), and β-copaen-4α-ol (4.32%) were identified as major compounds. E-Bisabol-11-ol (4.32%), E-bisabol-11-ol (0.92%), β-sinensal (0.57%), and α-himachalene oxide (0.19) were reported for the first time in its oil. Zerumbone was further isolated from the crude mixture of oil and identified by FTIR, NMR (1H, 13C, DEPT, HXCO, and HMBC), and mass (EI-MS) spectral data. Zerumbone showed promising antifungal activity against three phytopathogenic fungi, namely Rhizoctonia solani (EC50, 39.6 ppm), Sclerotium rolfsii (EC50, 59.3 ppm), and Macrophomina phaseolina (EC50, 147.4 ppm) compared with hexaconazole (EC50, 18.3, 13.4, and 4.5 ppm, respectively). Zerumbone has also shown concentration-dependent affect on seedling growth of Phalaris minor Retz., compared with control and Triticum aestivum L. (wheat). Zerumbone strongly suppressed the root and shoot growth of P. minor seedling at 1000 ppm compared with control and wheat. It exhibited no or less effect on the germination of seeds of T. aestivum.