Cytokinins are one of the major biologically active components in seaweed extracts. The effects of seaweed extract on turfgrass heat stress tolerance are not well understood. This study was to examine influences of a synthetic cytokinin-zeatin ribiside and cytokinins derived from seaweed extract on creeping bentgrass heat tolerance. The cytokinins at 3.5 mg/L were applied to the leaves of creeping bentgrass seedlings and the treated seedlings were grown in a hydroponic solution culture system at 35 C for 48 days. The grass treated with the natural and the synthetic cytokinins had greater visual quality, photochemical efficiency, chlorophyll content, superoxide dismutase activity, and less lipid peroxidation when compared to the non-treated control. The natural cytokinins also increased endogenous cytokinins. Our results suggest that heat tolerance enhancement upon application of seaweed extract may largely result from the component cytokinins.