The spike‐like substructure in picosecond optical pulses from gain‐switched multimode injection lasers is analyzed. A theoretical description based on the interference of the randomly phased longitudinal laser modes and the transient shift of the envelope of the mode spectrum under gain‐switched operation is presented. Single shot, streak camera records and autocorrelation traces are simulated, describing all essential experimental features of the substructure like its random variation from pulse to pulse and inherent periodicity.