Seagrass meadows provide crucial ecosystem services to the coastal zone but are threatened globally. Seagrass loss to date has mainly been attributed to anthropogenic activities that reduce light quantity (amount of photosynthetic photon flux density), such as dredging, flooding and eutrophication. However, light quality (wavelengths of light within the visible spectrum) is also altered by these anthropogenic stressors. This study addressed the effect of light quality changes on seagrasses. Aquarium-based experiments were conducted to determine whether the seagrass Halophila ovalis (R.Br.) Hook f. responds to different light quality treatments. Separate experiments were performed in which adults, seeds or seedlings were subjected to monochromatic light treatments in the blue (peak λ = 451 nm), green (peak λ = 522 nm), yellow (peak λ = 596 nm) and red (peak λ = 673 nm) wavelengths with a control of full-spectrum light (λ = 400 - 700 nm, at 200 μmol photons m-2 s-1). This study is unique in that it measured seagrass responses to light across several plant scales (physiology, productivity, morphology and biomass) as well as across life-history stages (seeds, seedlings, adults and flowering). Adult plants responded differently to seeds and seedlings but were generally consistent with terrestrial angiosperms: blue light decreased below-ground productivity; green light influenced morphology (through increased rhizome internode length); red light en - hanced seed germination and survival. The findings indicate that both natural and human-induced changes in light quality could significantly affect seagrass growth and reproduction. As a range of anthropogenic activities are currently contributing to the global losses of seagrasses, this research provides timely information on how light quality influences different seagrass life history stages.