Biosynthesis (also called biogenesis) is an enzyme-catalyzed process in cells of living organisms by which substrates are converted to more complex products. The biosynthesis process often consists of several enzymatic steps in which the product of one step is used as substrate in the following step. Examples for such multi-step biosynthetic pathways are those for the production of amino acids, fatty acids, and natural products. Biosynthesis plays a major role in all cells, and many dedicated metabolic routes combined constitute general metabolism.

The prerequisites for biosynthesis are precursor compounds, chemical energy (such as in the form ATP), and catalytic enzymes, which may require reduction equivalents (e.g., in the form of NADH, NADPH).

Commonly known complex products of biosynthesis include proteins, vitamins, and antibiotics. Most organic compounds in living organisms are built in biosynthetic pathways.