嘧啶二聚体（pyrimidine dimer，简称PD），是DNA或RNA中的相邻碱基在紫外线的诱导下产生共价键而形成的一种化合物，是突变产生的原因之一。嘧啶二聚体可在光或修复酶的作用下恢复成为单体。

Pyrimidine dimers are molecular lesions formed from thymine or cytosine bases in DNA via photochemical reactions. Ultraviolet light induces the formation of covalent linkages by reactions localized on the C=C double bonds. In dsRNA (double-stranded RNA), uracil dimers may also accumulate as a result of UV radiation. Two common UV products are cyclobutane pyrimidine dimers (CPDs, including thymine dimers) and 6,4 photoproducts. These premutagenic lesions alter the structure of DNA and consequently inhibit polymerases and arrest replication. Dimers may be repaired by photoreactivation or nucleotide excision repair, but unrepaired dimers are mutagenic. Pyrimidine dimers are the primary cause of melanomas in humans.