Heading date is a critical trait that determines cropping seasons and regional adaptability in rice (Oryza sativa). Research efforts during the last decade have identified some important photoperiod pathway genes that are conserved between Arabidopsis and rice. In this study, we identified a novel gene, Oryza sativa ELF3 (OsELF3), which is a putative homolog of the ELF3 gene in Arabidopsis thaliana. OsELF3 was required for the control of heading date under long-day conditions. Its Tos17-tagging mutants exhibited a delayed heading date phenotype only under long-day, but not short-day, conditions. OsELF3 was highly expressed in leaf blades, and the OsELF3 protein was localized in the nucleolus. An obvious diurnal rhythm of OsELF3 transcript level was observed, with a trough in the early day and a peak in the late night in wild-type plants. However, this expression pattern was disrupted in oself3 mutants. Further investigations showed that the expression of OsGI and Ghd7 was up-regulated in the oself3 mutant, indicating that OsELF3 acts as a negative regulator upstream of OsGI and Ghd7 in the flowering-time control under long-day conditions. The rhythmic expression of circadian clock-related genes, including some OsPRR members, was obviously affected in oself3 mutants. Our results indicated that OsELF3 acts as a floral activator in the long-day photoperiodic pathway via its crosstalk with the circadian clock in rice.