<p>Some carotenoids have a function to quench excitation energy of chlorophyll in photosynthetic apparatuses. This is a part of the photoprotective mechanism, non-photochemical quenching (NPQ), which prevents the inhibitory effects of high-light irradiation. In the green sulfur bacterium <i>Chlorobaculum tepidum</i>, which is a strictly anaerobic photosynthetic bacteria, a glycosylated carotenoid (Car-G) has been found to dissipate excitation energy of bacteriochlorophyll (BChl) <i>a</i> in the photochemical reaction center (RC) complex. In this study, we used ultrafast transient absorption spectroscopy to investigate excitation energy transfer (EET) mediated by Car-G in the reaction center complex of <i>C. tepidum</i>. Two spectral changes in carotenoids were identified when a core-antenna BChl <i>a</i> was selectively excited. One was sensitive to detergent replacement, while the other was not. The latter was the formation of a triplet-excited state of carotenoid (³Car) and observed when the primary electron donor accumulated in the oxidized state. The corresponding signal was absent in the RC complex from the Δ<i>cruC</i> mutant strain lacking Car-G. Kinetic analyses and structural insights revealed that ³Car is formed on a tightly-bound Car-G through triplet-to-triplet (TT)-EET from one of the excited states of the core-antenna BChl <i>a</i>. This TT-EET to ³Car is responsible for the anaerobic energy dissipation in the RC complex of <i>C. tepidum</i>.</p>