圆二色性（英语：Circular dichroism）是一种生物分子在光学上的特性。

一般而言，生物分子大多都具有手征性，也就是说它们在结构上有其他分子式相同但结构式不同的映射异构体(分为L型和D型)，除此之外它们在光学特征上也有所差异。

通常光源在未经特殊处理前，其偏振方向通常是呈各种方向散射，经过起偏器偏振化后，光波会变为偏振方向单一的光波（称为线性偏振光）。当此平面偏振光通过手征性生物分子后，会分成左旋和右旋两道圆偏振光，最后再经过一道偏光镜使其重合为一线性偏光。

由于手征性生物分子结构上的影响，而使得左旋与右旋圆偏振光在折射率上有所差异，因此在重合后会产生附加的相位差，从而使得射出的合成线偏光在角度上产生偏转。

Circular dichroism (CD) is dichroism involving circularly polarized light, i.e., the differential absorption of left- and right-handed light. Left-hand circular (LHC) and right-hand circular (RHC) polarized light represent two possible spin angular momentum states for a photon, and so circular dichroism is also referred to as dichroism for spin angular momentum. This phenomenon was discovered by Jean-Baptiste Biot, Augustin Fresnel, and Aimé Cotton in the first half of the 19th century. It is exhibited in the absorption bands of optically active chiral molecules. CD spectroscopy has a wide range of applications in many different fields. Most notably, UV CD is used to investigate the secondary structure of proteins. UV/Vis CD is used to investigate charge-transfer transitions. Near-infrared CD is used to investigate geometric and electronic structure by probing metal d→d transitions. Vibrational circular dichroism, which uses light from the infrared energy region, is used for structural studies of small organic molecules, and most recently proteins and DNA.