软流圈（Asthenosphere）是地球地幔的一部分弱塑性变形区域，位于岩石圈的下面、中间圈的上面，其深度下界在地表以下180-220km深处，其基部甚至在700km深处。

岩石圈与软流圈的边界，定义在1300°C等温线。此线以上的岩石圈为刚性变形，此线以下的软流圈为黏滞变形。地震波在软流圈的波速下降10%左右，形成一个低速区。这可能是由于软流圈存在部分熔融。大洋地壳下的岩石圈-软流圈边界比较浅（平均在60km深处）。在洋中脊，岩石圈-软流圈边界在大洋地壳下几千米处。

软流圈由J．巴拉鹿于1914年命名，来源于希腊语单词asthenēs(weak)与单词sphere的组合；推测为塑性状态的超铁镁物质；软流圈以上的地幔顶部为坚硬的岩石，与地壳的岩石合称为岩石圈；因为板块构造论的地幔对流体的运动就是在软流圈中进行，岩石圈板块在软流圈之上受到对流体的驱动而飘移，故称软流圈。

The asthenosphere (from Greek ἀσθενής asthenḗs 'weak' + "sphere") is the highly viscous, mechanically weak and ductilely deforming region of the upper mantle of the Earth. It lies below the lithosphere, at depths between approximately 80 and 200 km (50 and 120 miles) below the surface. The Lithosphere-Asthenosphere boundary is usually referred to as LAB. The asthenosphere is generally solid, although some of its regions could be melted (e.g., below mid-ocean ridges). The lower boundary of the asthenosphere is not well defined. The thickness of the asthenosphere depends mainly on the temperature. In some regions the asthenosphere could extend as deep as 700 km (430 mi). It is considered the source region of mid-ocean ridge basalt (MORB).