Floridean starch is the storage polysaccharide of red algae (Rhodophyta), the group that also yields the commercial gelling agents agar and carrageenan. Chemically it is an α-glucan built from glucose units joined mainly by α-1,4 linkages with α-1,6 branch points, which makes it structurally similar to glycogen, the animal storage carbohydrate, and to amylopectin. Unlike the starch of green plants, it is deposited in the cytosol rather than inside plastids. It serves as the alga’s energy reserve, accumulating during photosynthesis and being broken down when light or nutrients are scarce. While not itself a commercial food ingredient, understanding floridean starch matters to phycologists and to processors of red seaweeds, since its presence and behaviour influence the extraction and quality of agar and carrageenan.