Lysine (abbreviated as Lys or K) is an α-amino acid with the chemical formula HO₂CCH(NH₂)(CH₂)₄NH₂. It is an essential amino acid, which means that the human body cannot synthesize it. Lysine's codons are AAA and AAG. Lysine is a base, as are arginine and histidine. The ε-amino group often participates in hydrogen bonding and as a general base in catalysis.

Lysine is one of the 20 amino acids normally found in proteins. Its primary amine (4-aminobutyl)functional group is protonated at physiological pH (~7.4), allowing it to act as a donor in hydrogen bonding and/or as a general base in catalysis. Other amino acids with these properties include arginine and histidine. It is an essential amino acid, with a human nutritional requirement of 1–1.5 g daily.

One gram contains 366 mg L Lysine