截面二次轴矩（second axial moment of area），又称截面惯量，或截面对某一轴的惯性矩，通常是对受弯曲作用物体的横截面而言，是反映截面的形状与尺寸对弯曲变形影响的物理量。弯曲作用下的变形或挠度不仅取决于荷载的大小，还与横截面的几何特性有关。如工字梁的抗弯性能就比相同截面尺寸的矩形梁好。它和反映截面抗扭转作用性能的极惯性矩是相似的。

截面二次轴矩虽然也称“惯性矩”，但它和用以计算旋转物体角加速度的惯性矩（常称为转动惯量）是不同的两个概念。二者有相同的名称，相同的符号（是英文中惯性 inertia 的首字母），但依据上下文二者不致混淆。而且二者的因次或单位不同：二次轴矩的单位是长度的四次方，而后者的单位是长度的二次方乘以质量。

This article is about the geometrical property of an area, termed the second moment of area. For the moment of inertia dealing with the rotation of an object with mass, see mass moment of inertia.

For a list, see list of second moments of area.

The second moment of area, also known as moment of inertia of plane area, area moment of inertia, or second area moment, is a geometrical property of an area which reflects how its points are distributed with regard to an arbitrary axis. The second moment of area is typically denoted with either an for an axis that lies in the plane or with a for an axis perpendicular to the plane. In both cases, it is calculated with a multiple integral over the object in question. Its unit of dimension is length to fourth power, L.