Onion is one of the most important cash crops grown in Ethiopia. However, the production and productivity of the crop in the country is very low, which is associated with inappropriate agronomic practices including use of inappropriate seedling age and lack of seeds of improved and adaptable varieties. Therefore, to evaluate the effects of varieties and seedling age on growth and yield performance of onion, field experiment was carried out during 2019 irrigation season in Fogera district. The treatments consisted of three varieties of onion (Adama red, Bombay red and Nasik red) and 4 seedling ages (30, 40, 50 and 60 days), which were laid down in Randomized Complete Block Design with three replications. Seeds of onion varieties were obtained from Melkassa Agricultural Research Center. Phenological, growth and yield parameters were collected using standard procedures and the data were analyzed using SAS (Version 9.2) software. The results of the study revealed that both variety and seedling age very highly significantly (P < 0.001) influenced leaf number at 75 and 90 days after transplanting, fresh bulb weight, marketable bulb yield, unmarketable bulb yield and total bulb yield. Maturity days was very highly significantly (P<0.001) affected by the interaction effect of variety and seedling age. The interaction of Bombay red variety with 60 days seedling age gave the earliest maturity days (105.33 days) than other combinations. The highest leaf number at 90 days after transplanting (12.00), bulb weight (84.39 g) marketable bulb yield (29.97 t/ha) and total bulb yield (30.05 t/ ha) were recorded from Bombay red variety. 60 days old seedlings performed well in terms of all the tested growth and yield parameters including the highest marketable (32.03 t/ha) and total bulb yield (32.39 t/ha). Marketable bulb yield of onion was very strongly and positively (p<0.001) correlated with plant height, leaf length, number of leaves per plant, stand count, bulb diameter and bulb weight. Hence, using 60 days old seedlings and Bombay red variety can be recommended for the production of onion in the study area during irrigation season. However, further study has to be done under different seasons and locations to get forceful recommendation. Keywords: Adama red, Bombay red, Bulb yield, Marketable yield, Nasik red, Seedling age