Fifty corn poppy populations originating from three counties of northern Greece were evaluated for resistance to tribenuron. Twelve of the populations (six sampled from cereal fields and six from margins) were sampled from the county of Thessaloniki, 15 (13 sampled from wheat fields and two from margins) from Kilkis, and 23 (21 sampled from cereal fields and two from margins) from Serres. Fifty, 39, and 95% of the populations sampled from winter wheat fields originating from Thessaloniki, Kilkis, and Serres were resistant (R), respectively. However, all populations sampled from margins of the same areas were susceptible (S). All populations examined were susceptible to 2,4-D and bromoxynil. The level of resistance to tribenuron varied among populations with the herbicide dose required to reduce growth by 50% (GR50) ranging from 41 g ha−1 (R/S, resistance ratio 137) for the least resistant to over 720 g ha−1 (R/S greater than 2,400) for the most resistant populations. Fresh weight accumulation, seed production, and capsule number of eight R populations grown under field conditions were similar to those recorded for eight S populations originating from sites with high proximity of their respective R populations. However, the estimated mean growth rate (MGR) indicated significant differences due to the resistance trait and the population's origin. In particular, the R populations that originating from Thessaloniki and Serres had MGR 1.3 to 4.3 times lower than the respective S populations, whereas the R populations from Kilkis had similar or higher MGR values compared to the respective S populations. The populations with the highest R/S (greater than 2,400) had low MGR values, and the populations with R/S ranging from 1,437 to 2,227 had high MGR values.