Seed germination is one of the most important biological processes in the life cycle of plants, and temperature and water are the two most critical environmental factors that influence seed germination. In the present study, we investigated the roles of the plant hormone abscisic acid (ABA) and reactive oxygen species (ROS) in high temperature (HT) and drought-induced inhibition of rice seed germination. HT and drought stress caused ABA accumulation in seeds and inhibited seed germination and seedling establishment. Quantitative real-time polymerase chain reaction analysis revealed that HT and drought stress induced the expression of OsNCED3, a key gene in ABA synthesis in rice seeds. In addition, ROS (O and HO) and malondialdehyde contents were increased in germinating seeds under HT and drought stress. Moreover, we adopted the non-invasive micro-test technique to detect HO and Ca fluxes at the site of coleoptile emergence. HT and drought stress resulted in a HO efflux, but only drought stress significantly induced Ca influx. Antioxidant enzyme assays revealed that superoxide dismutase (SOD), peroxidase, catalase (CAT), and ascorbate peroxidase (APX) activity were reduced by HT and drought stress, consistent with the expression of OsCu/ZnSOD, OsCATc, and OsAPX2 during seed germination. Altogether, these results suggest that ABA and ROS accumulation under HT and drought conditions can inhibit rice seed germination and growth.