<p>Onion is the most important and widely cultivated bulb vegetable in Ethiopia. However, high postharvest losses during storage remain a major challenge to market supply and consumption. This study evaluated the effects of selected preharvest treatments on the shelf life and postharvest quality of onion varieties stored under diffused light storage conditions. Bulbs of three hybrid varieties (Red Coach, Russet, and Jambar) and one open-pollinated variety (Bombay Red), produced during the 2023 irrigation season at the Woramit Research and Training Subcenter of ARARI, were subjected to four preharvest treatments: toppling at 70% neck fall with irrigation, toppling at 70% neck fall without irrigation, toppling at 90% neck fall, and harvesting at 70% neck fall without toppling. Bulbs were stored for 4, 8, and 12 weeks in a diffused light storage facility at Bahir Dar University. Treatments were arranged in a 4 × 4 × 3 factorial RCBD with three replications, and data were analyzed using SAS 9.0. Variety, preharvest treatment, and storage duration significantly affected most postharvest quality parameters. Interaction effects between variety and preharvest treatment significantly influenced decay, sprouting, firmness, total soluble solids, weight loss, and marketable yield. Jambar and Russet bulbs subjected to toppling at 90% neck fall showed the lowest sprouting and weight loss, and the highest firmness and marketable bulb percentage throughout storage. In contrast, Bombay Red with toppling at 70% neck fall under irrigation exhibited the highest decay, sprouting, and weight loss, and the lowest firmness. Overall, the use of Jambar and Russet varieties combined with toppling at 90% neck fall is recommended to reduce postharvest losses and improve onion storability under diffused light conditions. Further studies across multiple seasons and locations, including the use of plant growth regulators such as malic hydrazide, are recommended.</p>