分子轨道理论（英语：Molecular orbital theory），简称MO理论，是处理双原子分子及多原子分子结构的一种有效的近似方法，是化学键理论的重要内容。它与价键理论不同，后者着重于用原子轨道的重组杂化成键来理解化学，而前者则注重于分子轨道的了解，即认为分子中的电子围绕整个分子运动。

计算化学中常以原子轨道线性组合近似来计算分子轨道波函数：

式中的c_ij系数可由将等式代入薛定谔方程以及应用变分原理求得。简单地讲，该方法意即，分子轨道由原子轨道组合而成。原子轨道波函数各乘以某一系数相加或相减，得到分子轨道波函数。组合时原子轨道对分子轨道的贡献体现在系数上，组合前后轨道总数不变。

In chemistry, molecular orbital (MO) theory is a method for determining molecular structure in which electrons are not assigned to individual bonds between atoms, but are treated as moving under the influence of the nuclei in the whole molecule. The spatial and energetic properties of electrons within atoms are fixed by quantum mechanics to form orbitals that contain these electrons. While atomic orbitals contain electrons ascribed to a single atom, molecular orbitals, which surround a number of atoms in a molecule, contain valence electrons between atoms. Molecular orbital theory, which was proposed in the early twentieth century, revolutionized the study of bonding by approximating the positions of bonded electrons—the molecular orbitals—as Linear Combinations of Atomic Orbitals (LCAO). These approximations are now made by applying the density functional theory (DFT) or Hartree–Fock (HF) models to the Schrödinger equation.