The β sheet (also β-pleated sheet) is the second form of regular secondary structure in proteins, only somewhat less common than the alpha helix. Beta sheets consist of beta strands connected laterally by at least two or three backbone hydrogen bonds, forming a generally twisted, pleated sheet. A beta strand (also β strand) is a stretch of polypeptide chain typically 3 to 10 amino acids long with backbone in an almost fully extended conformation. The higher-level association of β sheets has been implicated in formation of the protein aggregates and fibrils observed in many human diseases, notably the amyloidoses such as Alzheimer's disease.

In the most common usage, β strand refers to a single continuous stretch of amino acids adopting an extended conformation and involved in backbone hydrogen bonds to at least one other strand; by contrast, a β sheet refers to an assembly of at least two such β strands that are hydrogen-bonded (or H-bonded) to each other.

The first β sheet structure was proposed by William Astbury in the 1930s. He proposed the idea of hydrogen bonding between the peptide bonds of parallel or antiparallel extended β strands. However, Astbury did not have the necessary data on the bond geometry of the amino acids in order to build accurate models, especially since he did not then know that the peptide bond was planar. A refined version was proposed by Linus Pauling and Robert Corey in 1951.