Triterpenoids derived from plants are a promising class of natural antidepressants. This research focused on the therapeutic potential of quinoa saponin (QS) in alleviating lipopolysaccharide (LPS)-induced anxiety and depressive-like behaviors in mice. The most abundant saponin fraction, QS-3, was isolated from QS extracts, and its major saponin components and chemical structure were elucidated. Six pentacyclic triterpene saponins and three tetracyclic triterpene saponins were identified in QS-3, with phytolaccagenin and oleanolic acid being the dominant sapogenins. In vivo studies demonstrated that QS significantly mitigated LPS-induced anxiety and depressive-like behaviors in mice, enhanced the levels of neurotrophic proteins, key synaptic proteins, and neurotransmitters, and restored synaptic function and neuronal damage. Furthermore, QS inhibited neuroinflammation by curtailing the activity of the TLR4/MyD88/NF-κB pathway and modulating microglial phenotypes. Notably, QS also ameliorated colonic inflammation by promoting gut microbiota homeostasis and increasing short-chain fatty acids (SCFAs) production, which contributed to the improvement of anxiety and depressive behaviors in mice. Our findings suggest that QS holds potential as a natural dietary supplement for the treatment and prevention of anxiety and depression, possibly through its modulation of gut-brain axis dynamics and suppression of the activation of the TLR4/MyD88/NF-κB pathway.