Drought is a major environmental stress limiting growth of perennial grasses in temperate regions. Candidate gene association mapping provides a powerful tool for dissection of complex traits. We conducted candidate gene-association mapping of drought tolerance traits in 192 diverse perennial ryegrass (Lolium perenne L.) accessions from 43 countries. The panel showed significant variations in leaf wilting, leaf water content, canopy and air temperature difference, and chlorophyll fluorescence under well-watered and drought conditions across six environments. Five population structures were found in the mapping panel. Fourteen candidate genes involved in antioxidant metabolism, dehydration, and water movement across membrane and signal transduction were sequenced and linkage disequilibrium decay was close to 0.1 in less than 1 kb. Significant associations were identified between a putative LpLEA3 encoding late embryogenesis abundant group 3 protein and a putative LpFeSOD encoding iron superoxide dismutase and leaf water content, as well as between a putative LpCyt Cu-ZnSOD encoding cytosolic copper-zinc superoxide dismutase and chlorophyll fluorescence under drought condition. Four of these identified significantly associated single nucleotide polymorphisms from these three genes were also translated to amino acid substitutions in different genotypes. These results indicate that allelic variation in these genes may affect whole-plant response to drought stress in perennial ryegrass.