分子轨道（Molecular orbital，MO）是分子轨道理论的一个核心概念，指电子在分子中的运动区域，该概念首先由弗里德里希·洪德和罗伯特·桑德森·马利肯在1927-1928年引入。

电子在分子中的空间运动状态可以用分子轨道波函数（ψ，薛定谔方程的数学解）描述，借助Hartree-Fock方程或自洽场方法可对其作定量近似。

定性上看，分子轨道由原子轨道线性组合（LCAO-MO法）获得，组合后的分子轨道数目与组合前的原子轨道数目相等，经过键结与反键结的作用后，分子轨域能量高低重新排列。

In chemistry, a molecular orbital (MO) is a mathematical function describing the wave-like behavior of an electron in a molecule. This function can be used to calculate chemical and physical properties such as the probability of finding an electron in any specific region. The term orbital was introduced by Robert S. Mulliken in 1932 as an abbreviation for one-electron orbital wave function. At an elementary level, it is used to describe the region of space in which the function has a significant amplitude. Molecular orbitals are usually constructed by combining atomic orbitals or hybrid orbitals from each atom of the molecule, or other molecular orbitals from groups of atoms. They can be quantitatively calculated using the Hartree–Fock or self-consistent field (SCF) methods.