Pyrimidines are a class of bases found in nucleosides and nucleotides that make up nucleic acids. The pyrimidines cytosine and thymine are found in DNA, whereas cytosine and uracil are found in RNA.

In nucleic acids, the pyrimidines match up with specific purine bases. The matching between purines and pyrimidines forms "base-pairs" in which uracil (in RNA) or thymine (in DNA) pairs with adenine. Cytosine pairs with the base guanine in either nucleic acid.