Following the detection of fall armyworm Spodoptera frugiperda (J.E. Smith, Lepidoptera: Noctuidae) in Western Australia in early 2020 and the lack of government response action, we estimate the impact it is likely to have on the state's agriculture. A bioeconomic model is used to estimate cost and revenue implications for broadacre cropping and horticulture industries. We assume permanent S. frugiperda populations are likely to establish in areas of the state's north and mid-west over the next decade, and other regions may experience sporadic outbreaks over single seasons. Over 0.8 million hectares of host crops could be permanently affected, while sporadic outbreaks may affect a further 150,000 hectares. Expressed in Australian dollars (A$), S. frugiperda is likely to add a A$14.2–39.3 million burden to agricultural producers per annum by year 10 of the outbreak. Approximately 55% of these damage costs are attributable to yield loss and 45% to increased variable production costs.