ABSTRACT. – Boron (B) is a crucial micronutrient needed for plant growth. This study investigated the effect of B addition (0.5 mg kg–1-low, and 5 mg kg–1-high) on lettuce (Lactuca sativa L.) plants grown in the soil and hydroponically. The results showed that B addition causes a differential increase in B concentration in lettuce leaves depending on the cultivation system. In soil-grown plants, a statistically significant increase in lettuce B concentration was shown with increasing B treatments compared to the control. Moreover, the B concentrations measured in lettuce were within the optimal level for healthy lettuce. While hydroponically-grown plants showed a statistically significant increase in lettuce B concentration at high B treatment compared to the control and low B treatment. However, the high B treatment in hydroponics caused a very high B concentration in lettuce (196.4 mg kg–1) leading to toxicity expressed by the decrease in normalized difference vegetation index (NDVI) and chlorophyll content. Therefore, B concentration should be below 5 mg kg–1 for hydroponically-grown lettuce. Soil total and available B concentrations increased with B addition and considered a sufficient level. The other soil physicochemical parameters did not reveal a statistically significant difference with the B treatments, except a modest increase in pH, suggesting that the soil has a great homeostatic capacity. Additionally, the results showed that hydroponically-grown lettuce performs much better in terms of biomass production.