The world's population is expected to reach 9.7 billion by 2050, requiring a 60% increase in agricultural production. However, irrigation and agricultural activities contribute to water scarcity. This research aimed to evaluate the effects of different mulches and cultivars on onion growth, yield, and water productivity under supplementary irrigation at the Koga Irrigation Scheme in Ethiopia with a Randomized Complete Block Design and various data collection methods. The results indicated that the type of mulch significantly influenced onion growth parameters (at p<0.05), such as plant height, bulb diameter, and number of leaves per plant. Trash polyethylene mulches (PETM) performed exceptionally well, promoting the growth of both Nasik Red and Nafis cultivars. When compared to no mulch (NM) and maize cob mulch (MCM) treatments, Trash PETMs resulted in higher plant height, increased number of leaves per plant, longer leaf length, and larger bulb diameter, measuring 46.70cm, 4.60, 38.97cm, and 37.93mm, respectively. Nasik Red cultivars exhibited greater bulb diameter and leaf length, while Nafis cultivars excelled in the number of leaves per plant and plant height. The Nasik Red with PETM outperformed other mulch-cultivar interactions in plant height, increased number of leaves per plant, and bulb diameter, measuring 47.42cm, 4.67, and 38.15mm, respectively. PETM demonstrated significantly higher biomass yield, marketable bulb yield, water productivity, and lower non-marketable bulb yields compared to MCM and NM treatments, with values of 16810.2Kg/ha, 9396.88Kg/ha, 2.03Kg/m3, and 119.08Kg/ha, respectively. Nasik Red cultivars also exhibited higher biomass yield, total bulb yield, marketable bulb yield, water productivity, and lower non-marketable bulb yields than Nafis cultivars. The water productivity was significantly higher in the treatments using PETM with a value of 2.03 Kgm-3 compared to the NM-treated onion, which had the lowest CWP. Benefit-cost analysis revealed that the PETM was significantly superior to treatments with NM for both cultivar types. Overall, in similar agricultural settings, it is recommended that onion cultivators and policymakers should prioritize the selection of appropriate cultivars and mulch types. Keywords: onion, yield, water, Mulches, Cultivars, irrigation, bulb, water productivity.⁤