The increasing trend of extreme heat events poses serious implications for rice production in China, especially when it coincides with the heat-sensitive reproductive growth period (RGP) leading to high temperature-induced spikelet sterility. (2) Methods: This study analyzed the long-term meteorological data of Jiangsu province, located at lower reaches of Yangtze River Valley, to ascertain changing trends in temperature extremes. Moreover, stress days and nights were derived from temperature extremes, during RGP as a proxy of rice sterility in the province. The stress days were defined in three ways, with higher daily maximum (≥35 °C, hot day, HD), and minimum temperature (≥28 °C, hot night, HN) and days with both critical limits (severe hot day, SHD). (3) Results: The analysis of data revealed that a definite warming hiatus period in the 1970s and a transitional phase in 1980s were evident for daily maximum temperatures (TMX) in the province. Warm phases since 1990s were evident for both day and night temperature extremes. Asymmetric warming of TMX and daily minimum (TMN) temperatures reduced diurnal temperature range (DTR) over time. The slight negative trend for TMX and HD were observed, while trends for TMN, HN and SHD were strongly positive in the province with regional variations. (4) Conclusion: Observed increasing trends of TMN, HNs, and SHDs, as well as the decreasing DTR during rice RGP can be a threat to rice production in the province. Rice crop in the Southern Jiangsu is more vulnerable to higher temperature extremes while in the northern Jiangsu, decreasing DTR can be crucial. These findings could have useful implications for improving rice yield under changing climate.