Giant kelp is an ecologically and economically significant brown seaweed that creates biodiverse underwater forests, shoreline protection, commercially relevant compounds, and bioremediative opportunities for excess nutrient absorption. Anthropogenic disturbance and ocean warming threaten kelp forests along its habitat range, spanning much of the western coasts of North and South America as well as patches in South Africa and Australia. As a phototroph, anticipating and matching light cycles is integral for the timing of daily and seasonal physiology important to its fitness; however, these temporal dynamics and their mechanisms are not well characterized on a molecular level in any multicellular phototrophic protist. Here, we analyzed daily transcriptomic profiles of kelp in the field and under constant conditions in the lab. We found diel cycles in the field that persist under constant lab conditions, indicating regulation by an endogenous biological timekeeping mechanism, a phenomenon that evolved separately across the tree of life known as a circadian clock. These rhythms pertain to energy production and conversion, nitrogen metabolism, and amino acid transport and metabolism. We also identified 2 transcription factors as candidate clock-related genes with circadian domains common across the tree of life, one of which is an aureochrome (blue-light sensor) found throughout the brown algal clade. This first whole-transcriptome approach to circadian kelp biology sets up the foundation for understanding the seaweed physiology essential to successful management, restoration, and sustainable aquaculture efforts.