We examined the fruit productivity of vertically trained watermelon plants, focusing on leaf area, light reception, and field photosynthesis characteristics. We also examined the source?sink relationships of photosynthates during fruit development to characterize fruit productivity in vertical training. Furthermore, the fruit productivities of both the plant and unit land area were compared between vertical training systems and conventional, horizontal training systems. The fruit weight of both types of watermelon plants could be determined by the total leaf area, total solar radiation received, and photosynthetic production per plant. Regardless of the leaf position, most photosynthates of vertically trained watermelon plants during fruit development were distributed to the fruit, meaning the fruit weight could be determined by the above-mentioned factors. Photosynthates for roots were characteristically distributed from leaves on non-fruiting vines in plants with two vines and one fruit, and from limited lower leaves in plants with one vine and fruit respectively. Vertical training of watermelon plants increased the fruit yield per unit land area under higher planting density than horizontal training, with no loss of light-use efficiency.