A plant factory with artificial lighting is a closed plant production system designed to produce high yield with good quality. At the high plant densities used in plant factories, outer leaves beneath the dense canopy get insufficient light and senesce fast, resulting in poor yield. Methods that retard the senescence of outer leaves would improve yield and profitability. Here, we examined the effect of supplemental upward lighting with different wavelengths (blue, green, red, and white) and intensities on the retardation of outer leaf senescence, photosynthesis, and plant growth in lettuce (Lactuca sativa L. var romana). Supplemental upward lighting of any type retarded senescence and decreased waste, improving the marketable shoot biomass. Blue and red were most effective, as both are efficiently absorbed by chlorophyll. White, blue, and red, but not green, significantly increased the ascorbic acid content in the outer leaves. These results indicate that supplemental upward lighting with blue, red, or white light-emitting diodes (LEDs) can achieve high yields of lettuce with low waste and a high ascorbic acid content.