A ratchet mechanism for the headband of a protective helmet or other headgear allows for adjustment of the size and fit of the headband. The headband has overlapping rear end portions which are enclosed in a housing, which is comprised of an outer substantially arc-shaped housing section joined to a inner substantially arc-shaped housing section. Also contained with the housing sections is an adjustment mechanism adapted to cause lateral movement of the overlapping rear end portions of said headband with respect to one another. The outer and inner housing sections are substantially segmented into multiple discrete portions such that the respective housing sections are flexible along defined boundaries between the discrete portions to provide for a better and more comfortable fit.