Low-density lipoprotein (LDL) is one of the five major groups of lipoprotein. These groups, from least dense to most dense, are chylomicrons, very low-density lipoprotein (VLDL), intermediate-density lipoprotein (IDL), low-density lipoprotein and high-density lipoprotein (HDL). Lipoproteins transfer lipids around the body in the extracellular fluid. They can be sampled from blood and allow fats to be taken up by the cells of the body by receptor-mediated endocytosis. Lipoproteins are complex particles composed of multiple proteins.

LDL particles pose a risk for cardiovascular disease when they invade the endothelium and become oxidized, since the oxidized forms are more easily retained by the proteoglycans. LDL particles (though far different from cholesterol per se) are sometimes referred to as bad cholesterol because they can transport their content of lipid molecules into artery walls, attract macrophages, and thus drive atherosclerosis.

LDL particles are formed as VLDL lipoproteins lose triglyceride through the action of lipoprotein lipase (LPL) and they become smaller and denser (i.e. fewer fat molecules with same protein transport shell), containing a higher proportion of cholesterol esters. LDL lipoproteins interfere with the quorum sensing system that upregulates genes required for invasive Staphylococcus aureus infection.