This study was carried out to investigate the effects of irrigating lettuce (Lactuca sativa L. cv. Fun ly) with different concentrations of diluted seawater (0%, 2.5%, 5%, 10%, 15%, 20%) on the fresh yield, marketable yield and quality (DM, total soluble solids, titratable acidity, total sugar, vitamin C, NO3-N, protein, and total oxalate content). The experiments were conducted in a greenhouse in the autumn of 2012. The fresh yield, marketable yield, and DM of lettuce irrigated with 2.5% and 5% seawater were similar to those of control, but these parameters decreased in response to 10% seawater, and the lowest values were obtained in response to 20% seawater. The 2.5% seawater treatment had no effect on the vitamin C and NO3-N content, but both significantly decreased when lettuce was irrigated with seawater concentrations higher than 2.5%. Total soluble solids, total sugar, and protein content significantly increased in response to low salinity (2.5% and 5%) but decreased in response to increasing seawater stress. The titratable acidity values remained unchanged under the various saline conditions. Irrigation with diluted seawater did not affect the total oxalate content up to a concentration of 5%, but increasing the concentration of seawater above 5% increased oxalate content. The results of this study demonstrated that low concentrations of seawater are suitable for lettuce production and lettuce can be grown successfully using diluted seawater at concentrations of 2.5% and 5%.