Monoterpenes, the major constituents of essential oils, are known for their diverse biological activities. This study was conducted to assess the herbicidal effect of six monoterpenes viz. (R)-carvone, 1,8-cineole, cuminaldehyde, (S)-fenchone, geraniol, (S)-limonene, and (R)-linalool on barnyardgrass under laboratory and glasshouse conditions with a view to explore the possibility of their utilization for future weed management. The effect of monoterpenes on chlorophyll contents and total phenolic compounds was also evaluated. The inhibitory effects of monoterpenes on seed germination and seedling growth were tested at concentrations of 1, 2, 4, 6, and 8 mM. The results showed that geraniol and (R)-carvone caused greatest reduction of seed germination with complete inhibition at the concentrations > 2 mM. Similarly, these two compounds were the most potent inhibiters for root and shoot growth. In general, monoterpenes were less effective against seed germination than seedling growth. Furthermore, the inhibition of root growth by all compounds was greater than that of shoot growth. In foliar application treatments under glasshouse conditions, the monoterpenes reduced the fresh and dry weights, and shoot length of two-leaf stage barnyardgrass at concentrations of 1 and 2%. In addition, the tested monoterpenes caused phytotoxicity symptoms, mainly chlorosis and necrosis, followed by weed death. Complete weed control was observed in the treatments with 1 and 2% of geraniol, and 2% of cuminaldehyde. Further, a reduction of chlorophyll contents and total phenolic compounds of barnyardgrass leaves was noticed, indicating that the monoterpenes cause adverse effect on photosynthesis and weed metabolism. Based on the results of this study, it can be concluded that the monoterpenes, particularly geraniol, (R)-carvone, and cuminaldehyde, can be used as potential natural herbicides.