Temperatures and photoperiods can profoundly affect plant growth and development, and play vital roles in the local adaptation of plant species. Weedy rice (Oryza sativa f. spontanea) is a conspecific weed of cultivated rice, occurring in the same rice fields (sympatry) of early and late rice-cultivation seasons. In this study, the contrasting patterns of air-temperature and daylength variation were identified between the early and late seasons in Leizhou, Guangdong Province of China, where lower air-temperatures and longer daylengths in the early seasons were found with more stressful conditions for weedy rice growth and development. Significant differences in plant heights, the number of tillers per plant, flowering time, and reproductive traits were detected between the corresponding early- and late-season weedy rice populations of the same rice fields in the early-season common garden experiment. The populations collected from the early season showed higher plant heights, more tillers, and earlier flowering time than those from the late season. However, such differences were not detected in the late-season common garden experiment. In addition, evident local adaptation represented by the traits such as plant heights, flowering time, and reproductive traits were only detected in the early-season weedy rice populations. These results suggest that the early-season weedy rice populations may have evolved adaptive to the early seasons. Altogether, these findings provided evidence for phenotypical divergence between sympatric weedy rice populations, most likely resulted from the local adaptation to the early seasons in a stressful environment, supporting the theory of sympatric divergence in ecological speciation.