The antennal sensilla serve as a crucial olfactory organ, enabling insects to detect semiochemicals and adjust their host-seeking and oviposition behaviors accordingly. Monolepta signata (Oliver) (Coleoptera: Chrysomelidae), has emerged as a significant agricultural pest that affects key economic crops such as maize and cotton. Despite the development of various control methods based on volatile stimulation, there is still limited documentation on the sensilla involved in olfaction. In this study, the ultrastructure of the sensilla, especially the olfactory sensilla on the antennae of both males and females, was investigated with scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Three types of olfactory sensillum types, including trichodea, basiconica, and coeloconica, and four non-olfactory sensilla including chaetica, campaniformia, auricillica, and Böhm bristle were observed. Sensilla trichodea and basiconica on the antennae of M. signata were further classified into two subtypes according to their morphology. For the first time, the pores on the sensilla trichodea, basiconica, and coeloconica cuticular walls were observed in this species, suggesting that they are involved in semiochemical perception. This study contributes new insights into the olfactory system of M. signata, which can be integrated with other molecular, genetic, and behavioral research to establish a comprehensive understanding of its physiological functions.