In Timor-Leste, most paddy fields are abandoned after rice harvest due to limited water resources for another rice production cycle, particularly in lowland coastal areas. There is substantial scope for including legumes and other crops in the rice–fallow system in Timor-Leste. This study investigated the adaptation of grain legumes to terminal drought. The experiment was undertaken in 2018 and 2019 at field sites in Vemase and Laleia, respectively, on the northeast coast of Timor-Leste. The experiments used a split-plot design with two factors (water treatment and species) and three blocks (Vemase site) or four blocks (Laleia site). In 2018, the water treatments were well-watered control (W0), water withheld from flower initiation to maturity (W1), and water withheld after seedling establishment to maturity (W2). In 2019, the water treatments were well-watered control (W0) and water withheld from flower initiation to maturity (W1). Grain legumes were mungbean and soybean tested against grass pea (cv. Ceora), a well-known drought-adapted grain legume. The measured parameters included soil water content, crop phenology, plant growth and development, yield and yield components. The experiments revealed that mungbean is the most suitable grain legume crop after rice harvest under moderate drought conditions, while soybean is the preferred option under severe drought. Grass pea could be the best adaptive grain legume under severe drought in Timor-Leste when combined with the worsening conditions of climate change.