Croftonweed is a major invasive weed in China and, although of subtropical origin, has invaded into regions with colder climates. Freezing tolerances of nine croftonweed populations from different geographies were studied using a freezing injury index. Physiological responses to freezing temperature were determined to elucidate mechanisms of freezing tolerance. Plants from Baise, Guangxi (BSG), and Qujing, Yunnan (QJY), China, showed the most freezing injury symptoms, whereas plants from Huangguoshu, Guizhou (HGG), China, displayed the least. Under freezing stress, physiological changes, including increases in malondialdehyde (MDA) and total soluble protein contents; reductions in total soluble sugar, chlorophyll contents, and ratios of variable chlorophyll fluorescence to maximum chlorophyll fluorescence (Fv : Fm); and fluctuation of superoxide dismutase (SOD) activity, were observed among all nine populations. However, different degrees of physiological responses were found among populations with diverse low-temperature sensitivities. After 4 d of treatment at −5 C, MDA content increased 25-fold in leaves of the sensitive BSG population compared with untreated leaves, whereas a range of 0.8-fold to ∼5.3-fold increase was found in other populations. Total soluble protein content in leaves of the tolerant HGG population increased to the highest value among the nine populations. SOD activity of the freezing-sensitive BSG population decreased 36% of the control, whereas the tolerant HGG population reduced to 70%. Moreover, soluble sugar of the tolerant HGG population decreased 29%, less than the sensitive BSG population (87%). There were fewer declines in the percentages of chlorophyll content and Fv : Fm value in HGG than in BSG (less 44% and 32%). Freezing injury index had significant negative correlations with Fv : Fm values and chlorophyll contents (−0.619 and −0.622, respectively). These results suggest that croftonweed has evolved into different ecotypes with regard to freezing tolerance through physiological adaptation during their invasion of southwest regions of China. The freezing-tolerant croftonweed population would have more chances to invade distant northeastern areas in the future.