Grain number and seed-setting rate are components of crop yield. Cytokinin influences grain yield. However, emerging studies suggest that high cytokinin signals often lead to reduced branching or seed-setting rate, leading to reduced grain yield, although the mechanisms remain unclear. In this study, we identified and characterized the rice (Oryza sativa L.) gene LONELY GUY-LIKE 8 (LOGL8), based on analysis of the LOGL8-pm (promoter mutant of LOGL8) mutant, which harbors a T-DNA insertion in the promoter of this gene. The mutation in LOGL8-pm causes ectopic hyperexpression of LOGL8 in inflorescence organs, resulting in plants with smaller panicles and defective ovules lacking archesporial cells and integuments. Knockout of LOGL8 caused pollen abortion, leading to a reduced seed-setting rate. LOGL8 encodes a putative cytokinin-activating enzyme. Our results showed that LOGL8 directly catalyzes the biosynthesis of bioactive cytokinins. Therefore, we propose that the ectopic expression of LOGL8 disrupts cytokinin spatiotemporal distribution and causes inhibition of LONELY GUY (LOG), which affects panicle branching and female organ development. These findings reveal the important role of LOGL8 in male development, and highlight the delicate balance of local cytokinin levels during panicle branching and female organ development.