This study identifies a molecular marker for artemisinin-resistant *Plasmodium falciparum* malaria. Using whole-genome sequencing of an artemisinin-resistant parasite line from Africa and clinical isolates from Cambodia, the researchers found that mutations in the *PF3D7_1343700* kelch propeller domain (K13-propeller) are associated with *in vitro* and *in vivo* resistance to artemisinin. The K13-propeller mutations cluster in Cambodian provinces where resistance is prevalent, and their increasing frequency correlates with the spread of resistance in western Cambodia. The presence of mutant alleles, *in vitro* parasite survival rates, and *in vivo* parasite clearance rates are strongly correlated, indicating that K13-propeller mutations are important determinants of artemisinin resistance. The K13-propeller polymorphism is a useful molecular marker for large-scale surveillance to contain artemisinin resistance in the Greater Mekong Subregion and prevent its global spread.This study identifies a molecular marker for artemisinin-resistant *Plasmodium falciparum* malaria. Using whole-genome sequencing of an artemisinin-resistant parasite line from Africa and clinical isolates from Cambodia, the researchers found that mutations in the *PF3D7_1343700* kelch propeller domain (K13-propeller) are associated with *in vitro* and *in vivo* resistance to artemisinin. The K13-propeller mutations cluster in Cambodian provinces where resistance is prevalent, and their increasing frequency correlates with the spread of resistance in western Cambodia. The presence of mutant alleles, *in vitro* parasite survival rates, and *in vivo* parasite clearance rates are strongly correlated, indicating that K13-propeller mutations are important determinants of artemisinin resistance. The K13-propeller polymorphism is a useful molecular marker for large-scale surveillance to contain artemisinin resistance in the Greater Mekong Subregion and prevent its global spread.