Ionospheric ions (mainly H+, He+ and O+) escape from the ionosphere and populate the Earth’s magnetosphere. Their thermal energies are usually low when they first escape the ionosphere, typically a few eV to tens of eV, but are energized in their journey through the magnetosphere. The ionospheric population is variable, and it makes significant contributions to the magnetospheric mass density in key regions where magnetic reconnection is at work. Solar wind - magnetosphere coupling occurs primarily via magnetic reconnection, a key plasma process that enables transfer of mass and energy into the near-Earth space environment. Reconnection leads to the triggering of magnetospheric storms, aurorae, energetic particle precipitation and a host of other magnetospheric phenomena. Several works in the last decades have attempted to statistically quantify the amount of ionospheric plasma supplied to the magnetosphere, including the two key regions where magnetic reconnection proceeds: the dayside magnetopause and the magnetotail. Recent in-situ observations by the Magnetospheric Multiscale spacecraft and associated modelling have advanced our current understanding of how ionospheric ions alter the magnetic reconnection process at meso- and small-scales, including its onset and efficiency. This article compiles the current understanding of the ionospheric plasma supply to the magnetosphere. It reviews both the quantification of these sources and their effects on the process of magnetic reconnection. It also provides a global description of how the ionospheric ion contribution modifies the way the solar wind couples to the Earth’s magnetosphere and how these ions modify the global dynamics of the near-Earth space environment.