A synchrocyclotron is a special type of cyclotron, patented by Edwin McMillan in 1952, in which the frequency of the driving RF electric field is varied to compensate for relativistic effects as the particles' velocity begins to approach the speed of light. This is in contrast to the classical cyclotron, where this frequency is constant.

There are two major differences between the synchrocyclotron and the classical cyclotron. In the synchrocyclotron, only one dee (hollow "D"-shaped sheet metal electrode) retains its classical shape, while the other pole is open (see patent sketch). Furthermore, the frequency of oscillating electric field in a synchrocyclotron is decreasing continuously instead of kept constant so as to maintain cyclotron resonance for relativistic velocities. One terminal of the oscillating electric potential varying periodically is applied to the dee and the other terminal is on ground potential. The protons or deuterons to be accelerated are made to move in circles of increasing radius. The acceleration of particles takes place as they enter or leave the dee. At the outer edge, the ion beam can be removed with the aid of electrostatic deflector. The first synchrocyclotron produced 195 MeV deuterons and 390 MeV α-particles.