Soils under intensive agricultural practices such as those for growing vegetables in plastic greenhouses are an important anthropogenic source of nitrous oxide (N2O). Nitrous oxide emissions and measures to mitigate them through fertilizer N management have been less frequently studied than open field systems. The objectives of this study were to measure N2O emissions from vegetables under greenhouse conditions in Southern China and to investigate the effect of reducing the amount of applied synthetic N fertilizer compared with local practice. Results indicate that the average N2O-N flux during the growth of four vegetables (tomato, cucumber, celery and a second tomato crop) was 117.4 ± 9 μg N/m2/h, and the annual emission rate was 8.1 ± 0.6 kg/N/ha for local farms. Temperature was important with much lower emissions during the celery-growing season when soil and air temperatures were frequently <10 °C. Nitrous oxide emissions from the greenhouse vegetables were seven times greater than from the rice–wheat system in the same area and soils. Reducing the amount of applied synthetic N fertilizer by 40% relative to local farmers’ normal usage could reduce annual cumulative N2O emissions by 33% without any impact on crop yields.