Effects of extended photoperiods in red and blue LED supplemental lighting (SL) at constant photosynthetic photon flux density (PPFD) on growth, yield and quality of 'Patagonia' iceberg lettuce (Lactuca sativa var. capitata 'Iceberg') were evaluated. Different treatments were 14 hours of SL (T1), 16 hours of SL (T2) and 18 hours of SL (T3) under the similar ratio of 70:30 (R:B) at supplemented PPFD of 70 μmolm−2s−1 in comparison to treatment T4 i.e., No Supplemental Lighting (NSL) or control. The experiment was laid in a completely randomized design, and lettuce plants grown in soilless media in 100% complete nutrient solution under naturally ventilated greenhouse in winter season were planted in six replicates per treatment. Supplemental lighting was only used when the outside solar radiation dropped below 300 W/m2. The average temperature and relative humidity were within the range of 11.8–25 °C and 66.3–88.3%, respectively. The results indicate that growth parameters enhanced in respect of increased photoperiods. Yield was significantly different in all the treatments with highest value of 12.8 Kg/m2 under treatment T3 followed by 12.0 Kg/m2, 11.1 Kg/m2 and 10.3 Kg/m2 under T2, T1 and T4, respectively. Plants raised under SL treatments matured earlier in comparison to control. Treatment T3 significantly raised quality of lettuce in terms of L-ascorbic acid content, firmness, total phenolic content, DPPH radical scavenging activity and total soluble solids. Our findings demonstrate that extended photoperiods in supplemental LED lighting systems could improve growth, increase yield and enhance quality of iceberg lettuce grown in soilless media under protected cultivation. [ABSTRACT FROM AUTHOR]