下夸克（down quark）为第一代粒子，电荷为-(1/3)e ，是第二轻的夸克，质量并不是很确定，大约4-8MeV。根据标准模型理论它跟上夸克是组成核子的基本粒子；质子是由一个下夸克跟两个上夸克组成，则中子是个两下夸克跟个一上夸克组成（但核子的质量主要来源不是上、下夸克，而是把它们捆绑在一起的胶子。）

The down quark or d quark (symbol: d) is the second-lightest of all quarks, a type of elementary particle, and a major constituent of matter. Together with the up quark, it forms the neutrons (one up quark, two down quarks) and protons (two up quarks, one down quark) of atomic nuclei. It is part of the first generation of matter, has an electric charge of −⁄₃ e and a bare mass of 7000480000000000000♠4.8+0.5
−0.3 MeV/c. Like all quarks, the down quark is an elementary fermion with spin-⁄₂, and experiences all four fundamental interactions: gravitation, electromagnetism, weak interactions, and strong interactions. The antiparticle of the down quark is the down antiquark (sometimes called antidown quark or simply antidown), which differs from it only in that some of its properties have equal magnitude but opposite sign.