Onion is one of the most consumed vegetables all over the world, and Uruguay is not the exception. As a part of a global study of the pesticide intake of the Uruguayan population, we present the results of the evaluation of pesticide residues content in three onion varieties marketed in Montevideo. Samples of spring onion (Allium fistulosum), white and red onion (Allium cepa) were analyzed for 57 pesticide residues through gas chromatography coupled to tandem mass spectrometry. The compounds studied included the pesticides approved to be applied to onion plus the banned organochlorines. As onion is a bulb, the food contamination by non-approved and no longer used organochlorines cannot be dismissed because they are very persistent compounds that are still found in soils of old farms, where horticultural activities have been carried out for decades. The method will be useful for the monitoring of pesticde residues in onions markted in Uruguay, aiming to perfrmo risk assesment studies on pesticde exposure of the uruguayan population.