Non-native grass invasion has increased fuel loads and fire frequency in areas throughout the tropics, resulting in a non-native grass–wildfire cycle with negative impacts on native biodiversity and ecological processes. Megathyrsus maximus (guinea grass) invades dry and mesic ecosystems throughout the tropics, increasing fuel loads and wildfire intensity. Eradication of M. maximus is difficult, making effective wildfire management critical to the protection of adjacent developed areas and remnant native ecosystems. The use of domestic livestock grazing in non-native grass ecosystems may be effective at decreasing fine fuel loads and potential wildfire behaviour. Our objectives were to: (1) quantify live and dead fine fuel loads and moistures in a M. maximus–dominated ecosystem before and after cattle grazing, and (2) use these data to model potential wildfire behaviour in grazed and ungrazed M. maximus grasslands with the BehavePlus fire modelling system. Fine fuel loads and moistures, climate variables, and predicted wildfire behaviour were quantified at the same site (n = 1) over two 5-month periods (March–July 2009, ungrazed; March–July 2010, grazed) in the Wai‘anae Kai Forest Reserve on the Island of O‘ahu, Hawai‘i. Strong to conclusive evidence existed that cattle grazing in this system decreased dead and total fuel loads, but did not alter live fuel loads, or live and dead fuel moistures. Modelled wildfire behaviour under both low and average fuel moisture scenarios revealed that grazing decreased the potential rate of spread by 44–52% and flame length by 36–41%. These results demonstrate that cattle grazing may be an effective approach for reducing fuel loads and potential wildfire behaviour in non-native-dominated grasslands on tropical islands.