Land use and land cover changes (LUCC) is a key factor in determining regional vegetation greenness, impacting terrestrial carbon, water, and energy budgets. As a global hotspot of LUCC, Southeast Asia has experienced intensive cropland and plantation expansions in the past half-century, yet their impacts on regional greenness have not been elucidated. Here, we harmonized multiple land cover datasets, and used satellite-derived leaf area index (LAI) in combination with a machine learning approach to quantify the impacts of LUCC on vegetation greenness in insular Southeast Asia (i.e., Peninsular Malaysia, Sumatra, and Borneo islands). We found that regional LAI shows almost no trend (0.04 × 10^-2 m² m^-2 yr^-1) from 2000 to 2016, as a net effect of increased LAI (+5.71 × 10^-2 m² m^-2 yr^-1) due to CO2 fertilization, offset by decreased LAI mainly due to cropland expansion (-4.46 × 10^-2 m² m^-2 yr^-1). The impact of croplands on greenness in Southeast Asia contrasts with that in India and China. Meanwhile, oil palm expansion and climate change induced only small decreases in LAI in Southeast Asia (-0.41 × 10^-2 m² m^-2 yr^-1 and -0.38 × 10^-2 m² m^-2 yr^-1, respectively). Our research unveils how LAI changes with different processes of LUCC in Southeast Asia and offers a quantitative framework to assess vegetation greenness under different land use scenarios.