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Simple Summary: Worldwide, pest control involves extensive use of insecticides, which results in serious environmental pollution problems. On the other hand, insecticides can be recognized by proteins named CSPs in insects, which allow them to accurately respond to these environmental chemical signals for their survival, but the mechanism is poorly studied. Here, we report the crystal structure of the CSP8 protein from the tobacco cutworm Spodoptera litura, a major plant pest in Asia. We also studied its binding properties to compounds like rhodojaponin III, a non-volatile plant metabolite. Our studies showed that the protein binds to these molecules with different affinities and provided important insight into the molecular recognition mechanism of the sensory protein SlCSP8 and the CSP protein family in general. Spodoptera litura F. is a generalist herbivore and one of the most important economic pests feeding on about 300 host plants in many Asian countries. Specific insect behaviors can be stimulated after recognizing chemicals in the external environment through conserved chemosensory proteins (CSPs) in chemoreceptive organs, which are critical components of the olfactory systems. To explore its structural basis for ligand-recognizing capability, we solved the 2.3 Å crystal structure of the apoprotein of S. litura CSP8 (SlCSP8). The SlCSP8 protein displays a conserved spherical shape with a negatively charged surface. Our binding assays showed that SlCSP8 bound several candidate ligands with differential affinities, with rhodojaponin III being the most tightly bound ligand. Our crystallographic and biochemical studies provide important insight into the molecular recognition mechanism of the sensory protein SlCSP8 and the CSP family in general, and they suggest that CSP8 is critical for insects to identify rhodojaponin III, which may aid in the CSP-based rational drug design in the future. [ABSTRACT FROM AUTHOR]