Identifying the factors underlying severe COVID-19 in the host genetics is an emerging issue1-5. We conducted a genome-wide association study (GWAS) involving 2,393 Japanese COVID-19 cases collected in initial pandemic waves with 3,289 controls, which identified a variant on 5q35 (rs60200309-A) near DOCK2 associated with severe COVID-19 in younger (<65 ages) patients (nCase=440, odds ratio=2.01, P=1.2×10-8). This risk allele was prevalent in East Asians but rare in Europeans, showing a value of non-European GWAS. RNA-seq of 473 bulk peripheral blood identified decreasing effect of the risk allele on DOCK2 expression in younger patients. DOCK2 expression was suppressed in severe forms of COVID-19. Single cell RNA-seq analysis (n=61) identified cell type-specific downregulation of DOCK2 and COVID-19-specific decreasing effects of the risk allele on DOCK2 in non-classical monocytes. Immunohistochemistry of lung specimens from severe COVID-19 pneumonia showed suppressed DOCK2. Moreover, inhibition of DOCK2 function using CPYPP induced much more severe pneumonia in a Syrian hamster model of SARS-CoV-2 infection characterized as weight loss, lung edema, enhanced viral loads, impaired macrophage recruitment and dysregulated type I interferon responses. We conclude that DOCK2 plays an important role in the host immune response to SARS-CoV-2 infection and development of severe COVID-19, and could be further explored as a potential biomarker and/or therapeutic target.