A deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) sequence composed of two or more covalently linked nucleotides. Oligonucleotides are classified as deoxyribooligonucleotides or ribooligonucleotides. Fragments containing up to 50 nucleotides are generally termed oligonucleotides, and longer fragments are called polynucleotides. Gene synthesis from synthetic deoxy oligo nucleotides is now routinely used to prepare genes and modified genes for proteins having potential clinical applications. Oligonucleotides have also been used to diagnose genetic disorders and bacterial or viral infections. Oligonucleotides are sometimes referred to as oligos.

A deoxyribooligonucleotide consists of a 5-carbon sugar called deoxyribose joined covalently to phosphate at the 5′ and 3′ carbons of this sugar to form an alternating, unbranched polymer. A ribooligonucleotide consists of a similar repeating structure where the 5-carbon sugar is ribose. Chemically synthesized oligonucleotides of predetermined sequence have proven to be very useful for studying a large number of biochemical processes. In the 1960s, these compounds were used to decipher the genetic code. Later, chemically prepared deoxyoligonucleotides were joined to form genes for transfer RNAs.