An antiproton beam was a type of weapon, a directed energy weapon that utilized antiproton subatomic particles, a basic component of antimatter that was the antiparticle of the proton.

An antiproton beam was used as a mounted starship weapon on the doomsday machine planet killer The machines highly refined technology was able to fire beams of pure antiproton, a fact which Federation scientists found astounding. (TOS episode & novelization: The Doomsday Machine)

An antiproton beam could also find a cloaked Romulan ship, and increase a starship's shields by a factor of 10. (DS9 episode: "Defiant"; VOY episode: "The Q and the Grey")