The metabolic profile of seaweeds can fluctuate depending on environmental factors and biotic/abiotic stresses. Osmundea pinnatifida is a red alga, known as Pepper Dulse in the UK, harvested from the wild and sold as seasoning due to its unique peppery taste. This paper highlights the seasonal variation in the compositional profile of biomass harvested from a single location over 12 months, linking this to variation in flavour. Hydrophilic interaction chromatography with tandem mass spectrometry (HILIC-PDA-MS) analysis putatively identified 33 major components. Some of these have been noted in other Osmundea species (e.g. chilenones) or identified as osmoregulators in other seaweeds (e.g. mycosporines, betaines or sugar-glycerol components). Central metabolites were identified (e.g. amino or organic acids), as were others not previously recorded in seaweeds. The metabolites varied in abundance across the seasons and could be allocated into five trends those that decreased in summer/increased in winter, increased or decreased in autumn, increased in summer/decreased in winter, those which did not vary, and those with no apparent pattern. Components were identified that increased in abundance in winter when the flavour of Pepper Dulse is more potent. Many of these components were extracted under aqueous conditions that replicate those in the mouth and could therefore contribute to the flavour of this seaweed. This information increases our knowledge about the biochemical composition and its seasonal variation of Osmundea pinnatifida providing insights on compounds that might be related to its taste, thus providing information relevant to future commercialization and harvest management of this species.