Mistletoes alter host tree physiology affecting important processes such as host growth and reproduction. However, studies examining the effects of mistletoe infection on host reproductive performance in semi-arid savannas remain scarce. Here, we investigated how mistletoe infection affected the reproduction of sweet thorn (Vachellia karroo Hayne), a major browse tree in semi-arid savanna, southwest Zimbabwe. Mistletoe infection reduced host reproductive performance, with increasing mistletoe abundance associated with declines in flower bud production and pod size (length, width, and diameter), while uninfected trees produced larger pods than mistletoe-infected trees. Host tree architecture also influenced reproduction. Tree height and canopy area generally enhanced reproductive output, whereas increasing tree diameter was associated with reduced seedbank abundance, pod size, and seed production. Notably, these effects were context dependent, as heavy mistletoe infection amplified the negative effects of tree diameter, but strengthened the positive effects of canopy area and height, indicating that the consequences of parasitism depends on the interaction between host size and parasite load. These findings demonstrate that mistletoe impacts on host reproduction extend beyond direct resource competition, influencing regeneration potential and highlighting the importance of host–parasite interactions in shaping savanna woody plant dynamics.