The osmopriming technique can reduce the period between sowing and the emergence of seedlings in the field, as well as favor seed performance under stress conditions. This study aimed to evaluate the effect of osmopriming on the physiological performance and antioxidative enzymatic activity of sunflower seeds with different vigor levels and exposed to thermal stress. Three sunflower seed lots of the cultivar Hélio 250 were used. Initially, the seeds were evaluated by germination and vigor tests to characterize the lots. Subsequently, they were primed in a polyethylene glycol 6000 solution at -2.0 MPa and 15 °C for 8 h. Then, the primed and unprimed seeds were tested for physiological quality (germination, first germination count, percentage and emergence speed index of seedlings, and seedling dry matter) and determination of the activity of the enzymes superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and peroxidase (POX) under three temperatures: 15 °C (sub-optimal), 25 °C (optimal), and 35 °C (supra-optimal). The physiological tests allowed classifying lots 1, 2, and 3 into three different vigor levels, i.e., high, medium, and low, respectively. Osmopriming favored the performance of sunflower seeds in terms of germination and vigor at all the analyzed temperatures. This effect was more pronounced in lots of lower physiological quality at sub-optimal and supra-optimal temperatures. Sub-and supra-optimal temperatures led to a reduction in the physiological performance of seeds, mainly in less vigorous lots. In general, osmopriming favored an increase in the activity of the enzymes SOD, CAT, POX, and APX, mainly in low vigor seeds exposed to sub and supra-optimal temperatures. Osmopriming of sunflower seeds in PEG 6000 at -2.0 MPa for 8 hours is efficient to improve the performance of less vigorous lots under stress due to the sub- and supra-optimal temperatures, favoring an increase in the activity of enzymes of the antioxidative system.