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With global warming, high temperature stress has become a main threat to the growth of cool-season turfgrasses, including perennial ryegrass. As one of the conserved plant microRNA families, miR408s are known to play roles in various abiotic stresses, including cold, drought, salinity, and oxidative stress, but no report, thus far, was found for heat. Here, perennial ryegrass plants overexpressing rice Os-miR408 were used to investigate the role of miR408 in plant heat tolerance. Both wild type (WT) and miR408 transgenic perennial ryegrass plants (TG) were subjected to short-term heat stress at 38 °C for 72 h (experiment 1) or at 42 °C for 48 h (experiment 2), and then let recover for 7 days at optimum temperature. Morphological changes and physiological parameters, including antioxidative responses of TG and WT plants, were compared. The results showed that miR408 downregulated the expression of two putative target genes, PLASTOCYANIN and LAC3. Additionally, overexpression of Os-miR408 improved thermo-tolerance of perennial ryegrass, demonstrated by lower leaf lipid peroxidation and electrolyte leakage, and higher relative water content after both 38 and 42 °C heat stresses. In addition, the enhanced thermotolerance of TG plants could be associated with its morphological changes (e.g., narrower leaves, smaller tiller angles) and elevated antioxidative capacity. This study is the first that experimentally reported a positive role of miR408 in plant tolerance to heat stress, which provided useful information for further understanding the mechanism by which miR408 improved plant high-temperature tolerance, and offered a potential genetic resource for breeding heat-resistant cool-season turfgrass in the future. [ABSTRACT FROM AUTHOR]