The proton-proton chain reaction (also known as the PP chain) is one of two fusion reactions by which stars convert hydrogen to helium, the other being the CNO cycle. The proton-proton chain is more important in stars the size of the Sun or less.

The first step involves the fusion of two hydrogen nuclei ¹H (protons) into deuterium ²H, releasing a positron as one proton changes into a neutron, and a neutrino. To overcome the electromagnetic repulsion between two hydrogen nuclei requires a large amount of energy, and this reaction takes an average of 10 billion years to complete. Because of the slowness of this reaction the Sun is still shining; if it where faster, the Sun would have exhausted its hydrogen long ago.

The neutrinos detected from the sun are significantly below what the proton-proton calculations predict, resulting in what is known as the solar neutrino problem. Observations of pressure waves in the sun, known as helioseismology have indicated that the pressures and temperatures in the sun are very close to the pressures and temperatures predicted, assuming our understanding the proton-proton chain is correct. This has led astrophysicists to believe that the resolution of the solar neutrino problem lies in unexpected behavior of the neutrinos after they are produced.