古地磁学（或称古磁学）研究史前地质地球磁场变化与强度的一门科学，作为地磁学的一个分支。该学门研究之目的，主要在得知地球形成时残留于岩层的磁场消息，再配合其他数据来进行统计方法分析，可用来从事多种地球科学上之研究。 岩石中留存的磁性，这些岩石包含磁铁矿和赤铁矿等含铁矿石。在岩石形成过程中，或由于岩浆沉积作用、结晶作用，或化学反应致使矿物颗粒中内部磁场被地球磁场磁化而造成岩石的磁性。测量岩石中「化石磁」的方向，就可能测定岩石形成时的古纬度和当时地极的位置。

Paleomagnetism (or Palaeomagnetism in the United Kingdom) is the study of the record of the Earth's magnetic field in rocks, sediment, or archeological materials. Certain minerals in rocks lock-in a record of the direction and intensity of the magnetic field when they form. This record provides information on the past behavior of Earth's magnetic field and the past location of tectonic plates. The record of geomagnetic reversals preserved in volcanic and sedimentary rock sequences (magnetostratigraphy) provides a time-scale that is used as a geochronologic tool. Geophysicists who specialize in paleomagnetism are called paleomagnetists.