The Cosmotron was a particle accelerator, specifically a proton synchrotron, at Brookhaven National Laboratory.

It was the first accelerator that was able to produce all positive and negative mesons known to exist in cosmic rays.

The name chosen for the synchrotron was Cosmitron (representing an ambition to produce cosmic rays) but was changed to Cosmotron to sound like the cyclotron.

The beam size of 64 × 15 cm and an energy goal of about 3 GeV determined the machine parameters.

The straight sections without magnets were worrisome because there was no focusing and the betatron oscillations would change suddenly and might swing wildly.