抗磁性（Diamagnetism，亦作反磁性）是一些类别的物质，当处在外加磁场中，会对磁场产生的微弱斥力的一种磁性现象。

抗磁性的成因，是当物质处在外加磁场中，外加磁场使得物质电子轨道（更精确的讲法：轨域）运动产生改变的连带效应。当施加一外源磁场B时，会对运动中的电子（电荷q）产生了磁力F：F = qv × B。此力改变了电子所受的向心力，使得电子轨道运动或是加速，或是减慢。电子速度因此受到改变，而连带改变了其与外加磁场相反方向上的轨道磁矩。

考虑两个电子轨域：一个顺时针运动，一个逆时针运动。一进入页面方向的外加磁场会使顺时针转动电子的向心力增加，而使其自页面出来方向上的磁矩增加。同样的外加磁场则会使逆时针转动电子的向心力减少，而使其进入页面方向上的磁矩减少。两者的改变都与进入页面方向的外加磁场相抗衡。然而，外加磁场对于多数日常物质所感生的磁矩却非常小，因此净效应会是一种斥力。

Diamagnetic materials create an induced magnetic field in a direction opposite to an externally applied magnetic field, and are repelled by the applied magnetic field. In contrast, the opposite behavior is exhibited by paramagnetic materials. Diamagnetism is a quantum mechanical effect that occurs in all materials; when it is the only contribution to the magnetism the material is called a diamagnet. Unlike a ferromagnet, a diamagnet is not a permanent magnet. Its magnetic permeability is less than μ₀, the permeability of vacuum. In most materials diamagnetism is a weak effect, but a superconductor repels the magnetic field entirely, apart from a thin layer at the surface.