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Our understanding of the biological role of the Y chromosome remains limited. Here, we systematically profile germline Y haplogroups and somatic loss of the Y chromosome (LOY) in 122,683 East Asian males from BioBank Japan and 181,472 European males from the UK Biobank. A phenome-wide scan uncovers male-specific genetic regulation of complex traits, including pleiotropic effects of the Japanese-specific haplogroup D on height and type 2 diabetes (T2D). LOY increases T2D risk in East Asians but is associated with reduced T2D risk in Europeans. In East Asians, LOY contributes to T2D incidence particularly among males with lower polygenic risk scores, providing a compensatory explanation for disease risk beyond germline genetics. Incorporating sex-chromosome variation improves polygenic prediction of T2D risk in both sexes. Single-cell analyses reveal cell type-specific accumulation of LOY across tissues and disease contexts, with LOY in pancreatic β cells potentially impairing glucose metabolism. Our study demonstrates the clinical relevance of Y chromosome variation for diabetes risk prediction and management.