Carotenoids play indispensable roles in photosynthetic light-harvesting systems by mediating both light harvesting and photoprotection, with their excited-state relaxation pathways critically governing the efficiency of energy transfer to (bacterio)chlorophylls. However, whether a distinct intermediate state exists between the optically allowed S₂ (1Bu⁺) state and the optically forbidden S₁ (2Ag^-) state, as well as its physical origin, has long remained controversial. In this study, we investigate the excited-state structural dynamics of spheroidene, a carotenoid with a conjugation length of N = 10, in solutions using femtosecond stimulated Raman spectroscopy in conjunction with transient absorption spectroscopy. Our experiment results reveal a distinct S_X intermediate with 3Ag^- symmetry located between the S₂ and S₁ states, whose emergence is accompanied by ultrafast twisting of the polyene backbone. To further elucidate the relationship between polyene backbone twisting and S_X state formation, we tuned the viscosity of n-hexane by varying the temperature. The lifetimes of the S₂ and S_X states show pronounced viscosity dependence, whereas the S₁ state dynamics remain largely unaffected. These results demonstrated that the S_X state is a distinct electronic state with 3Ag^- symmetry, arising from a photoinduced polyene backbone twisting that triggers an inversion of the excited-state energy levels. This work resolves the long-standing controversy over the nature of the S_X intermediate in spheroidene and provides direct experimental evidence for its structural origin. More broadly, it offers a mechanistic basis for the high efficiency of carotenoid-to-bacteriochlorophyll energy transfer in natural light-harvesting systems, in which such structurally distorted intermediates may play an essential role.