An oligonucleotide, usually between 18 and 25 nucleotides in length, used in the amplification of specific DNA sequences using the polymerase chain reaction (PCR). A PCR reaction typically contains two primers, one binding in the "forward" orientation to the 5' end of the target DNA sequence, and the other binding in the "reverse" orientation to the 3' end of the target. During PCR, the double-stranded template DNA is melted into single strands, and the primers then anneal to the complementary sequence of the target DNA. An enzyme called DNA polymerase then binds to these primers and proceeds to race along the template DNA strand, using free nucleotides in the reaction mixture to generate a complementary strand that incorporates the primer at its 5' end.

The wide usage of PCR amongst modern biological scientists has generated a massive market for primers. These are now rapidly synthesised and delivered by large companies on request.