We demonstrate the existence of a prethermal dynamical phase transition (DPT) for fluctuating optical beams propagating in nonlinear dispersive media. The DPT can be probed by suddenly changing in time the dispersion and nonlinearity parameters of the medium (thus realizing a "temporal interface"), a procedure that emulates a quench in a massive $\varphi^4$ model. Above a critical value of the quench identifying the transition, the fluctuating beam after the temporal interface is characterized by a correlation length that diverges algebraically at the transition. Below the critical quench, the beam exhibits an algebraic relaxation and a self-similar scaling. Our analysis also reveals a dimensional cross-over of the critical exponent, a characteristic feature of the optical DPT.