In biochemistry, non-coded, non-proteinogenic, or "unnatural" amino acids are those not naturally encoded or found in the genetic code of any organisms. Despite the use of only 23 amino acids (21 in eukaryotes) by the translational machinery to assemble proteins (the proteinogenic amino acids), over 140 natural amino acids are known and thousands of more combinations are possible. Several non-proteinogenic amino acids are noteworthy because they are:

• intermediates in biosynthesis
• post-translationally incorporated into protein
• possess a physiological role (e.g. components of bacterial cell walls, neurotransmitters and toxins)
• natural and man-made pharmacological compounds
• present in meteorites and in prebiotic experiments (e.g. Miller–Urey experiment)