The proton (Greek πρῶτον / proton "first") is a subatomic particle with an electric charge of one positive fundamental unit (1.60217653(14)×10−19 C), a diameter of about 1.65×10−15 m, and a mass of 938.272309(28) MeV/c2 (1.007276466(13) u, 1.6726×10−27 kg), or about 1836 times the mass of an electron.

Protons are observed to be stable and their theoretical minimum half-life is 1×1036 years. Grand unified theories generally predict that proton decay should take place, although experiments so far have only resulted in a lower limit of 1035 years for the proton's lifetime. In other words, proton decay has never been witnessed.

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