What particle is the antiparticle of the proton?

The particle that is the antiparticle of the proton is the antiproton. It has the same mass and spin as a proton but the opposite electric charge and opposite magnetic moment.

A proton is made from two up quarks and one down quark. An antiproton is made from the corresponding antiquarks: two anti-up quarks and one anti-down quark. Like other matter–antimatter pairs, a proton and antiproton can annihilate when they meet, converting their mass and kinetic energy into other particles.

The antiproton was predicted by the development of relativistic quantum mechanics and first confirmed experimentally in 1955 by Emilio Segrè and Owen Chamberlain at the Bevatron accelerator. They received the 1959 Nobel Prize in Physics for the discovery.

A common mix-up is the antineutron, which is the antiparticle of the neutron, not the proton. A positron is also antimatter, but it is specifically the antiparticle of the electron.