Abstract: Endosymbionts are prevalent in insects, establishing close associations with their hosts that range from mutualism to parasitism. Endosymbionts provide hosts with essential nutrients, enhance immunity, and regulate reproduction, thereby profoundly influencing host fitness and adaptive evolution. Such impacts can drive genetic divergence and even speciation over time. Consequently, endosymbionts are critical to understanding the formation of cryptic insect species, which are morphologically indistinguishable yet genetically distinct. Elucidating the mechanisms underlying cryptic speciation remains a central challenge in evolutionary biology, with endosymbionts increasingly recognized as key drivers in this process. In this review, we summarize the fundamental characteristics of insect endosymbionts and their multifaceted roles in host biology. We conceptualize endosymbionts as a critical evolutionary force, focusing on the mechanisms by which they drive cryptic speciation, including ecological adaptation divergence, genetic integration, and reproductive manipulation. By integrating current evidence and elucidating underlying mechanisms, this review presents a framework for understanding endosymbiont-mediated cryptic speciation in insects. Research in this field holds great promise for revealing fundamental evolutionary processes and informing novel strategies for biodiversity conservation and pest management.