Although silicon (Si) is not an essential element for the majority of plant species, it plays an important role in plant resistance to a range of abiotic and biotic stresses. In this paper, we review the role of Si in regulating host plant resistance to herbivorous insects. Si can enhance resistance to insect herbivores in the majority of plant species, which depends on the sources and the application modes of Si fertilizer. Si can enhance constitutive resistance of plants to herbivorous insects by increasing hardness and abrasiveness of plant tissues and/or reducing digestibility. This leads to a reduction in insect growth and reproduction with a concomitant reduction in crop damage. Si content, sediment position and alignment in plants affects the subsequent strength of constitutive resistance. In addition, Si can regulate induced direct plant defenses as well as indirect defenses. Induced direct defense comprises producing toxic substances, producing localized hypersensitive response and systemic acquired resistance, and releasing toxic compounds and defensive proteins, which slow herbivore growth and development. Induced indirect defense is realized through the emission of volatile compounds that attract parasitic and/or predatory natural enemies of the herbivorous insects.