The rise in global temperatures is predicted to decrease the yield of important cereal crops such as wheat and barley. In barley (a model cereal crop), warm temperatures reduce yield by accelerating the time to flowering and reducing the number of florets per plant. Interestingly, this is only observed in long-day conditions. Currently, there have been no reports of genes or molecular pathways involved in temperature dependent flowering in the cereals. PHOTOPERIOD DEPENDENT 1 (PPD1) the major determinant of photoperiod dependent flowering in the cereals, is required for the acceleration of flowering at warm temperatures. Barley near isogenic lines carrying a non-functional ppd1 allele, flowered at the same time when grown in cool or warm temperatures. In warm temperatures, PPD1 accelerates both the transition to, and the early phases of reproductive development. Analysis of transcriptomic data from barley plants grown in cool and warm temperatures, identified other candidate genes that may be important for temperature dependent flowering. This study is the first to identify a gene involved in temperature dependent flowering in the cereals. It is an important first step in understanding the molecular pathways that regulate temperature dependent flowering. This will be vital to prevent yield losses in important cereal crops such as wheat and barley in a warming global environment.