The asthenosphere (from Ancient Greek ἀσθενός (asthenós) 'without strength') is the mechanically weak and ductile region of the upper mantle of Earth. It lies below the lithosphere, at a depth between c. 80 and 200 km (50 and 120 mi) below the surface, and extends as deep as 700 km (430 mi). However, the lower boundary of the asthenosphere is not well defined.

The asthenosphere is almost solid, but a slight amount of melting (less than 0.1% of the rock) contributes to its mechanical weakness. More extensive decompression melting of the asthenosphere takes place where it wells upwards, and this is the most important source of magma on Earth. It is the source of mid-ocean ridge basalt (MORB) and of some magmas that erupt above subduction zones or in regions of continental rifting.

The asthenosphere is a part of the upper mantle just below the lithosphere that is involved in plate tectonic movement and isostatic adjustments. It is composed of peridotite, a rock containing mostly the minerals olivine and pyroxene. The lithosphere-asthenosphere boundary is conventionally taken at the 1,300 °C (2,370 °F) isotherm. Closer to the surface at lower temperatures, the mantle behaves rigidly; deeper below the surface at higher temperatures, the mantle moves in a ductile fashion. The asthenosphere is where the mantle rock most closely approaches its melting point, and a small amount of melt is likely to present in this layer.

Seismic waves pass relatively slowly through the asthenosphere compared to the overlying lithospheric mantle. Thus, it has been called the low-velocity zone (LVZ), although the two are not strictly the same; the lower boundary of the LVZ lies at a depth of 180 to 220 kilometers (110 to 140 mi), whereas the base of the asthenosphere lies at a depth of about 700 kilometers (430 mi). The LVZ also has a high seismic attenuation (seismic waves moving through the asthenosphere lose energy) and significant anisotropy (shear waves polarized vertically have a lower velocity than shear waves polarized horizontally). The discovery of the LVZ alerted seismologists to the existence of the asthenosphere and gave some information about its physical properties, as the speed of seismic waves decreases with decreasing rigidity. This decrease in seismic wave velocity from the lithosphere to the asthenosphere could be caused by the presence of a very small percentage of melt in the asthenosphere, though since the asthenosphere transmits S waves, it cannot be fully melted.

In the oceanic mantle, the transition from the lithosphere to the asthenosphere (the LAB) is shallower than for the continental mantle (about 60 km in some old oceanic regions) with a sharp and large velocity drop (5–10%). At the mid-ocean ridges, the LAB rises to within a few kilometers of the ocean floor.

The upper part of the asthenosphere is believed to be the zone upon which the great rigid and brittle lithospheric plates of the Earth's crust move about. Due to the temperature and pressure conditions in the asthenosphere, rock becomes ductile, moving at rates of deformation measured in cm/yr over lineal distances eventually measuring thousands of kilometers. In this way, it flows like a convection current, radiating heat outward from the Earth's interior. Above the asthenosphere, at the same rate of deformation, rock behaves elastically and, being brittle, can break, causing faults. The rigid lithosphere is thought to "float" or move about on the slowly flowing asthenosphere, enabling isostatic equilibrium and allowing the movement of tectonic plates.

The asthenosphere extends from an upper boundary at approximately 80 to 200 km (50 to 120 miles) below the surface to a lower boundary at a depth of approximately 700 kilometers (430 mi).

The lithosphere-asthenosphere boundary (LAB) is relatively sharp and likely coincides with the onset of partial melting or a change in composition or anisotropy. Various definitions of the boundary reflect various aspects of the boundary region. In addition to the mechanical boundary defined by seismic data, which reflects the transition from the rigid lithosphere to ductile asthenosphere, these include a thermal boundary layer, above which heat is transported by thermal conduction and below which heat transfer is mainly convective; a rheological boundary, where the viscosity drops below about 10²¹ Pa⋅s; and a chemical boundary layer, above which the mantle rock is depleted in volatiles and enriched in magnesium relative to the rock below.