A disintegrin and metalloproteinase with thrombospondin motifs 13 (ADAMTS13) cleaves von Willebrand factor, reducing its prothrombotic activity. The genetic determinants of ADAMTS13 activity remain unclear. We performed a genome-wide association study of ADAMTS13 activity in the Rotterdam Study, a population-based cohort study. We used imputed genotypes of common variants in a discovery sample of 3443 individuals and replication sample of 2025 individuals. We examined rare exonic variant associations in ADAMTS13 in 1609 individuals using an exome array. rs41314453 in ADAMTS13 was associated with ADAMTS13 activity in both our discovery (β, −20.2%; P = 1.3 × 10⁻³³) and replication sample (P = 3.3 × 10⁻³⁴), and explained 3.6% to 6.5% of the variance. In the combined analysis of our discovery and replication samples, there were 2 further independent associations at the ADAMTS13 locus: rs3118667 (β, 3.0; P = 9.6 × 10⁻²¹) and rs139911703 (β, −11.6; P = 3.6 × 10⁻⁸). In addition, rs10456544 in SUPT3H was associated with a 4.2 increase in ADAMTS13 activity (P = 1.13.6 × 10⁻⁸). Finally, we found 3 independent associations with rare coding variants in ADAMTS13: rs148312697 (β, −32.2%; P = 3.7 × 10⁻⁶), rs142572218 (β, −46.0%; P = 3.9 × 10⁻⁵), and rs36222275 (β, −13.9%; P = 2.9 × 10⁻³). In conclusion, we identified rs41314453 as the main genetic determinant of ADAMTS13 activity, and we present preliminary findings for further associations at the ADAMTS13 and SUPT3H loci.