ABSTRACT Redroot pigweed (Amaranthus retroflexus L.) is a troublesome weed infesting soybean (Glycine max [L.] Merr.) productions in China. One redroot pigweed population, collected from Heilongjiang (HLJ) Province, China, was suspected to be resistant to thifensulfuron-methyl and fomesafen. The other one redroot pigweed population, collected from Shandong (SD) Province, was susceptible. The study aimed to characterize the level of thifensulfuron-methyl and fomesafen resistance using HLJ population and identify the potential resistance mechanisms to thifensulfuron-methyl. The sensitivity to other herbicides with and without the same target site was also evaluated. Acetolactate synthase (ALS) gene sequencing revealed that Trp574Leu or Ala205Val amino acid substitution were present in the HLJ population. Whole-plant herbicide bioassays showed that, compared with SD population, HLJ population displayed high level of resistance to thifensulfuron-methyl and moderate resistance to fomesafen. The 50% growth reduction (GR50) value of thifensulfuron-methyl with malathion pretreatment was reduced by 23%, suggesting that both target-site resistance and non-target-site resistance mechanisms were present in thifensulfuron-methyl resistance of redroot pigweed. Cross-resistant patterns showed that the HLJ population evolved resistance to pyrithiobac-sodium, pyroxsulam, imazethapyr and fluoroglycofen, but susceptible to bentazone.