<p>This paper is concerned with critical generalized transition waves for reaction–diffusion systems in time heterogeneous media. Focusing on the classical Fisher–KPP equation, we develop a novel proof technique to establish the existence of critical generalized transition waves. Our approach crucially relies on constructing appropriate sub- and supersolutions for the critical case, coupled with a standard limiting argument to derive wave existence. Notably, the proposed methodology extends beyond this specific model and is applicable to coupled systems and nonlocal dispersal problems, as demonstrated through a reaction-diffusion epidemic model and a nonlocal dispersal Fisher–KPP equation.</p>