The R process (R for rapid) is a neutron capture process for radioactive elements which occurs in high neutron density, high temperature conditions. Contrast with P and S-process. In the r-process nuclei are bombarded with a large neutron flux to form highly unstable neutron rich nuclei which very rapidly decay to form stable neutron rich nuclei.

The site of the r process is believed to be supernova. However, the abundance of r-process elements requires that either only a small fraction of supernova return r-process elements to the outside or that each supernova only contributes a very small amount of r-process elements.

Due to the much higher neutron flux in this process (on the order of 10²² neutrons per cm² per second), the rate of isotopic formation is much faster than the beta decays which follow, meaning that this process "runs up" along the neutron drip line, with the only two hold-ups being closed neutron shells extending the time it takes to create new isotopes, and the degree of nuclear stability in the heavy-isotope region, which terminates the r-process when such nuclei become readily unstable to spontaneous fission (currently believed to be in the region of A = 270 - or roughly in the Rutherfordium - Darmstadtium area of the periodic table).

Elemental abundance peaks show some support for the idea of rapid neutron capture and delayed beta emission, as the r-process peaks are at about 10 amu below those of the s-process peaks (which occur exactly at closed neutron shells), indicating that the "run up" along the neutron drip line reaches closed neutron shells but with sufficient proton deficiency to make the peaks resolvable.